Auto merge of #117868 - ferrocene:pa-omit-git-hash, r=Mark-Simulacrum

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 rust-lang/cargo#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.