re_build_build_info: PR builds fail in CI

[jleibs]

See for example:
https://github.com/rerun-io/rerun/actions/runs/5024217902/jobs/9009702716?pr=2161

Specific error:

  --- stderr
  thread 'main' panicked at 'Failed to run 'git ["symbolic-ref", "--short", "HEAD"]':

  fatal: ref HEAD is not a symbolic ref

  ', crates/re_build_build_info/src/lib.rs:103:5
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Error: The process '/usr/local/cargo/bin/cargo' failed with exit code 101

I believe is another problem stemming from similar conditions as what caused:

• Bacon failing to run due to custom build command #2155

In this particular case, during certain pull-request builds, the build does not take place on the branch commit, but rather an ephemeral merge-commit derived from merging the PR into main.

However, regardless, re_build_build_info needs to be robust to all manner of weird git environments.

I was hoping merging main into the branch would prevent this from happening but it seems not to be the case.

[emilk]

Already broken in #2173

[emilk]

This was because I pushed a ill-thought-through assert to the PR in question. I'll do something better in #2173 instead