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 Job ID Name Coverage
  Build
passed #1716
librem5
build-with-xwayland

00:01:24

passed #1717
librem5
build-without-xwayland

00:01:22