[cabal-7825] Add $CABAL env variable for external commands

[yvan-sraka]

This is a follow-up of #9063

Checklist:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.

QA notes:

I tested it as followed, on my machine I have 2 scripts:

• cabal-foobar:

  #! /usr/bin/env bash
  env | grep CABAL

• test-ext-cmd.sh

  #! /usr/bin/env bash
  PATH="$PATH:." cabal run cabal-install foobar

Then, e.g., as expected:

% ./test-ext-cmd.sh
CABAL=/Users/yvan/IOHK/cabal/dist-newstyle/build/aarch64-osx/ghc-9.2.8/cabal-install-3.11.0.0/x/cabal/build/cabal/cabal

[mpickering]

Closer.. but setting env like this overrides the whole environment, you need to get the current environment first using getEnvironment.

[mpickering]

I have written some simple tests for the external command interface (which would fail with this MR). Perhaps I should commit those first and then we can rebase this MR on top?

[yvan-sraka]

@mpickering AFAIU this PR could be closed since in #9412 you:

• Set the $CABAL environment variable to the current executable path
  • This allows external commands to be implemented by calling $CABAL,
    which is strongly preferred to linking against the Cabal library as
    there is no easy way to guantee your tool and cabal-install link
    against the same Cabal library.