ConfigFile/T8487/cabal.test.hs is fragile to changing compiler

This test modifies `--config-file`, which overrides the means the testsuite used to specify the compiler which cabal-install should use.

* If you use a `-w` flag to choose which ghc to compile the `cabal-tests` executable with.
* If you pass `--with-ghc=...` when running the tests

This doesn't show up in CI as we only test cabal-install against the ghc it was bootstrapped against.

I more robust way to write the test is to perhaps augment the `testUserCabalConfigFile`, which is local to each test, rather than override it completely. However, I will leave that for now.