common.sh: backport loopback_partscan retry to 77

BUG=chromium:1009559
TEST=tryjob

Change-Id: Ia5ed20e823f769332a419eaddf8b5932d8c13d64
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1833061
Reviewed-by: Jason Clinton <jclinton@chromium.org>
Commit-Queue: George Engelbrecht <engeg@google.com>
Tested-by: George Engelbrecht <engeg@google.com>
1 file changed