Confusing build-id date in about:preferences in alphas

After !487 (merged) we have started showing the correct version of Tor Browser.

However, when Firefox code detects that is an alpha (because of the 'a' in the version string) shows the build date.

We have our own system for that date (for reproducibility I think?), but it doesn't mean much for the users (e.g., it's July 2022 for 12.5a1).

We should either hide the date, or move to one that is more meaningful for users (e.g., the date of the commit we're building; we should modify RBM to expose it to a variable if needed).