Nix code for integration tests of Ormolu are now moved out of the main Nix file.
Due to the built-in Git awareness of flakes, the .git directory is no longer available, so gitrev can't work. Hence, we are now passing in the current commit rev (if it exists) as an environment variable at build time.
The IOG binary cache is specified in the flake, see nixConfig in flake.nix. This way, one does no longer have to globally configure it.
I included .envrc files for nix-direnv for convenience, but this is not super important.
Migrate to GH Actions.
Removed Ormolu Live for now as it won't build on the GH Actions runners due to low RAM. It will be replaced by a WASM-based alternative in #964.
.gitdirectory is no longer available, sogitrevcan't work. Hence, we are now passing in the current commit rev (if it exists) as an environment variable at build time.nixConfiginflake.nix. This way, one does no longer have to globally configure it..envrcfiles for nix-direnv for convenience, but this is not super important.