Environment files get written to $TMP when using cabal scripts

Running `cabal run --write-ghc-environment-files=always` on a cabal script doesn't actually write a GHC environment file.

It would be great if it did, since it would mean that after the first run, straightforward calls to `ghc` and `runghc` would just work, and `haskell-language-server`, for one, would be able to load the file without requiring any setup.

Is there any fundamental reason why this shouldn't just work?

I've actually ended up writing a hacky little script to parse the `build-depends` field and pass all the package names to `cabal install --package-env . --lib`, but it would be nice to have a proper solution.