Randomize test-ghc-env to avoid conflicts between parallel jobs

[zyla]

Since the build temporary directory is shared between parallel jobs, we can't use
the same file name for the test-ghc-env for all of them. This PR uses a random name for each parallel job to avoid conflicts.

Fixes #5024

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

[ulidtko]

The ghcEnvRandomId line interjects the preceding multiline comment explaining env variables and the next line, let setEnv ... — to which that comment is relevant. Please move somewhere else.

Perhaps like this?

                -- see e.g. https://github.com/doctest/issues/119
                -- also we set HASKELL_DIST_DIR to a package dist directory so
                -- doctest will be able to load modules autogenerated by Cabal
                let setEnv f pc = modifyEnvVars pc $ \envVars ->
                      Map.insert "HASKELL_DIST_DIR" (T.pack $ toFilePath buildDir) $
                      Map.insert "GHC_ENVIRONMENT" (T.pack f) envVars
+               ghcEnvRandomId <- liftIO (randomIO :: IO Int)
-                   fp = toFilePath $ eeTempDir ee </> testGhcEnvRelFile
+               let fp = toFilePath (eeTempDir ee </> testGhcEnvRelFile) <> show ghcEnvRandomId

Further, it's not so hard to stay within typed Path, just import addExtension:

-               ghcEnvRandomId <- liftIO (randomIO :: IO Int)
-               let fp = toFilePath $ eeTempDir ee </> testGhcEnvRelFile
+               randomSuffix <- ("-"<>) . show <$> liftIO (randomIO :: IO Int)
+               fp <- toFilePath <$> addExtension randomSuffix (eeTempDir ee </> testGhcEnvRelFile)

[mpilgrem]

Thanks! @zyla and @ulidtko, I have merged this.