update-git-snapshot: fix git invokations

`git co` is not always valid on Debian systems.
2 jobs for f/fix_update in 2 minutes and 46 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build-with-xwayland #1716
librem5

00:01:24

passed build-without-xwayland #1717
librem5

00:01:22