workflows: Avoid cloning the project repo in prune-dist

There is no need to clone the project repository just to determine the
SHA of current main. Use `git ls-remote` for that. Check for both "main"
and "master" to keep this in sync with cockpit.git and other projects.

Taken from cockpit-project/cockpit#15748

Author: martinpitt

.github/workflows/prune-dist.yml

@@ -10,9 +10,6 @@ jobs:
   run:
     runs-on: ubuntu-latest
     steps:
-      - name: Clone repository
-        uses: actions/checkout@v2
-
       - name: Set up configuration and secrets
         run: |
           printf '[user]\n\tname = Cockpit Project\n\[email protected]\n' > ~/.gitconfig
@@ -28,14 +25,15 @@ jobs:
         run: |
           set -ex
 
-          HEAD=$(git rev-parse HEAD)
+          # head commit SHA of default branch
+          HEAD=$(git ls-remote 'https://github.com/${{ github.repository }}' main master | cut -f1 | head -n1)
 
           cd dist-repo
           now="$(date +%s)"
           for tag in $(git tag -l); do
               tag_time="$(git show -s --format=%at $tag)"
               if [ "$tag" = "sha-${HEAD}" ]; then
-                  echo "$tag refers to current project HEAD, keeping"
+                  echo "$tag refers to current project default branch HEAD, keeping"
               elif [ $((now - tag_time)) -ge 604800 ]; then
                   echo "$tag is older than 7 days, deleting..."
                   git push origin ":$tag"