maintainer/scripts/check-cherry-picks: fix calling from outside nixpkgs

The CI job calls this as trusted/maintainers/..., i.e. with a working
directory outside the checkout. The git commands inside the script
assume to be inside the checkout, though, so let's force that.

(cherry picked from commit 642de212a6)

maintainers/scripts/check-cherry-picks.sh

@@ -8,6 +8,9 @@ if [ $# != "2" ] ; then
   exit 2
 fi
 
+# Make sure we are inside the nixpkgs repo, even when called from outside
+cd "$(dirname "${BASH_SOURCE[0]}")"
+
 PICKABLE_BRANCHES=${PICKABLE_BRANCHES:-master staging release-??.?? staging-??.??}
 problem=0