Add REPO_SKIP_SELF_UPDATE check in sync

The command _PostRepoFetch will try to self update
during repo sync. That is beneficial but adds
version uncertainty, fail potential and slow downs
in non-interactive scenarios.

Conditionally skip the update if env variable
REPO_SKIP_SELF_UPDATE is defined.

A call to selfupdate works as before, meaning even
with the variable set, it will run the update.

Change-Id: Iab0ef55dc3d3db3cbf1ba1f506c57fbb58a504c3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/439967


Tested-by: Fredrik de Groot <fredrik.de.groot@haleytek.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>