fix: Restore pip-compile invocation

In eb9ee31, pip-compile was changed to
no longer cd into the requirements directory. The reason for this change
was because of the use of `ONESHELL` which causes diverging behavior in
environments with make versions that don't yet support this mode.

This commit reverts the removal of the `cd` invocations, and
additionally removes the `ONESHELL` option which should make behavior
aligned across dev envs.

GNUmakefile

@@ -18,7 +18,6 @@ MAKEFLAGS     += --warn-undefined-variables
 MAKEFLAGS     += --no-builtin-rules
 SHELL         := bash
 .SHELLFLAGS   := -euxo pipefail -O globstar -c
-.ONESHELL:
 .SILENT:
 .SUFFIXES:
 
@@ -86,9 +85,9 @@ cleanest: cleaner
 requirements: export CUSTOM_COMPILE_COMMAND='make requirements'
 requirements:
 	pip install --upgrade pip setuptools pip-tools
-	pip-compile --upgrade --resolver=backtracking requirements/requirements.in -o requirements/requirements.txt
-	pip-compile --upgrade --resolver=backtracking requirements/requirements-dev.in -o requirements/requirements-dev.txt
-	pip-compile --upgrade --resolver=backtracking requirements/requirements-typing.in -o requirements/requirements-typing.txt
+	cd requirements && pip-compile --upgrade --resolver=backtracking requirements.in -o requirements.txt
+	cd requirements && pip-compile --upgrade --resolver=backtracking requirements-dev.in -o requirements-dev.txt
+	cd requirements && pip-compile --upgrade --resolver=backtracking requirements-typing.in -o requirements-typing.txt
 
 .PHONY: schema
 schema: against := origin/main