let the branch exist already

ci-tools/update-nixpkgs

@@ -4,10 +4,12 @@
 set -eux -o pipefail
 
 TARGET_BRANCH="nixpkgs-upgrade-$(date +%Y-%m-%d)"
-git checkout -b "${TARGET_BRANCH}"
 
 echo '{}' > morph/grid/local/public-keys/users.nix
 nix-build -A morph -o result-before
+
+git checkout "${TARGET_BRANCH}" || git checkout -b "${TARGET_BRANCH}"
+
 update-nixpkgs
 nix-build -A morph -o result-after
 diff=$(nix --extra-experimental-features nix-command store diff-closures ./result-before/ ./result-after/)