Cabal: Expose flag assignment to configure script (#8565)

* Cabal: Expose flag assignment to configure script

Here we extend the `build-type: configure` mechanism to allow the
configure script to inspect the flag assignment via the `CABAL_FLAGS`
environment variable.

As suggested in #8564.

* Fix typo in change log

* Make new test failures more informative

* Fix test

flags are no longer called `a` and `b`.

* Add warning for conflicting flag name env vars

* Convert haddocks to regular comments for local vars

Pitty haddocks on local vars broke doc tests CI job.

* Fix warning and, hopefully, tests

Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>