Wrong environment passed to happy build tool

[hsyl20]

Describe the bug

This was initially reported in #10072 (comment) but I'm not sure it's the same issue so I don't want to hijack Sebastian's ticket.

1. I have happy 2.0.2 globally installed (/usr/bin/happy)
2. For some reason Cabal puts happy 2.1.3 in the build plan and builds it
3. But then my globally installed happy 2.0.2 is called with the environment variable happy_lib_datadir set to /path/to/happy-lib-2.1.3/data, hence it uses the wrong HappyTemplate.hs and the generated code doesn't compile.

To Reproduce

Steps to reproduce the behavior:

In GHC's source tree:

	# Preparing source files...
	mkdir -p _build/stage0/src/
	cp -rf ./libraries   _build/stage0/src/
	cp -rf ./compiler    _build/stage0/src/libraries/ghc
	cp -rf ./ghc   	     _build/stage0/src/ghc-bin
	cp -rf ./utils       _build/stage0/src/
	
	## Substituting variables
	cp _build/stage0/src/ghc-bin/ghc-bin.cabal{.in,}
	cp _build/stage0/src/libraries/ghc/ghc.cabal{.in,}
	cp _build/stage0/src/libraries/ghc/GHC/CmmToLlvm/Version/Bounds.hs{.in,}
	cp _build/stage0/src/libraries/ghc-boot/ghc-boot.cabal{.in,}
	cp _build/stage0/src/libraries/ghc-boot-th/ghc-boot-th.cabal{.in,}
	cp _build/stage0/src/libraries/ghc-heap/ghc-heap.cabal{.in,}
	cp _build/stage0/src/libraries/ghci/ghci.cabal{.in,}
	cp _build/stage0/src/utils/ghc-pkg/ghc-pkg.cabal{.in,}
	
	sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/ghc-bin/ghc-bin.cabal
	sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/ghc-bin/ghc-bin.cabal
	sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/libraries/ghc/ghc.cabal
	sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/libraries/ghc/ghc.cabal
	sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/libraries/ghc-boot/ghc-boot.cabal
	sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/libraries/ghc-boot/ghc-boot.cabal
	sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/libraries/ghc-boot-th/ghc-boot-th.cabal
	sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/libraries/ghc-boot-th/ghc-boot-th.cabal
	sed -i 's/@Suffix@//' _build/stage0/src/libraries/ghc-boot-th/ghc-boot-th.cabal
	sed -i 's/@SourceRoot@/./' _build/stage0/src/libraries/ghc-boot-th/ghc-boot-th.cabal
	sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/libraries/ghc-heap/ghc-heap.cabal
	sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/libraries/ghc-heap/ghc-heap.cabal
	sed -i 's/@ProjectVersionForLib@/9.13/' _build/stage0/src/libraries/ghc-heap/ghc-heap.cabal
	sed -i 's/@ProjectVersion@/9.13/' _build/stage0/src/libraries/ghci/ghci.cabal
	sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/libraries/ghci/ghci.cabal
	sed -i 's/@ProjectVersionForLib@/9.13/' _build/stage0/src/libraries/ghci/ghci.cabal
	sed -i 's/@ProjectVersion@/9.13/'       _build/stage0/src/utils/ghc-pkg/ghc-pkg.cabal
	sed -i 's/@ProjectVersionMunged@/9.13/' _build/stage0/src/utils/ghc-pkg/ghc-pkg.cabal
	sed -i 's/@ProjectVersionForLib@/9.13/' _build/stage0/src/utils/ghc-pkg/ghc-pkg.cabal
	
	sed -i 's/@LlvmMinVersion@/13/' _build/stage0/src/libraries/ghc/GHC/CmmToLlvm/Version/Bounds.hs
	sed -i 's/@LlvmMaxVersion@/20/' _build/stage0/src/libraries/ghc/GHC/CmmToLlvm/Version/Bounds.hs
	
	# Building...
	mkdir -p _build/stage0/cabal/
	mkdir -p _build/stage0/bin/
	
	$(CABAL) build --project-file=cabal.project-stage0 \
	  ghc-bin:ghc ghc-pkg:ghc-pkg genprimopcode:genprimopcode deriveConstants:deriveConstants \
	  -j --builddir=_build/stage0/cabal/

-- cabal.project-stage0
packages:
  ./_build/stage0/src/ghc-bin/
  ./_build/stage0/src/libraries/ghc
  ./_build/stage0/src/libraries/directory/
  ./_build/stage0/src/libraries/file-io/
  ./_build/stage0/src/libraries/filepath/
  ./_build/stage0/src/libraries/ghc-platform/
  ./_build/stage0/src/libraries/ghc-boot/
  ./_build/stage0/src/libraries/ghc-boot-th/
  ./_build/stage0/src/libraries/ghc-heap
  ./_build/stage0/src/libraries/ghci
  ./_build/stage0/src/libraries/os-string/
  ./_build/stage0/src/libraries/process/
  ./_build/stage0/src/libraries/semaphore-compat
  ./_build/stage0/src/libraries/time
  ./_build/stage0/src/libraries/unix/
  ./_build/stage0/src/libraries/Win32/
  ./_build/stage0/src/utils/ghc-pkg
  ./_build/stage0/src/utils/hsc2hs
  ./_build/stage0/src/utils/unlit
  ./_build/stage0/src/utils/genprimopcode/
  ./_build/stage0/src/utils/deriveConstants/

benchmarks: False
tests: False
allow-boot-library-installs: True

program-options
  ghc-options: -fhide-source-paths -j

package *
  library-vanilla: True
  shared: False
  executable-profiling: False
  executable-dynamic: False
  executable-static: False
  ghc-options: -fhide-source-paths -j

package ghc-boot-th
  flags: +bootstrap

-- package genprimopcode
--   flags: -build-tool-depends

allow-newer: ghc-boot-th

Expected behavior

It should build without failing.

System information

• Operating system: linux
• Fails with cabal-install 3.14.1.0 and HEAD. Works with cabal-install 3.12.1.0

Additional context

I've bisected and the commit introducing the issue is 8bdda9c (cc @sheaf).

[andreabedini]

Definitely the exe dependencies have to be added to PATH (at the front). What is globally available should not influence the build.