Set `CFG_OMIT_GIT_HASH=1` during builds when `omit-git-hash` is enabled This environment variable will allow tools like Cargo to disable their own detection when `omit-git-hash` is set to `true`. I created this PR because of https://github.com/rust-lang/cargo/pull/12968. There is not a dependency between the two PRs, they can land in any order. They just won't do anything until both of them are merged into the repo.