sync: fix os.environ logic errors

This is a dict to index, not a function to call.

Change-Id: I0117eeaaa8b2ef4762ab6f0d22f9ffdaee961f52
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/258132


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>