.github: revise weekly check

For details, see https://stackoverflow.com/a/67527144/1708249

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/weekly.yml

@@ -10,22 +10,18 @@ on:
 jobs:
   schedule:
     runs-on: ubuntu-latest
+    outputs:
+      run: ${{ steps.check.outputs.run }}
     steps:
-      - name: Create cache file
+      - uses: actions/checkout@v3
+      - name: Check for any commits since last week
+        id: check
         run: |
-          mkdir weekly
-          echo ${{ github.sha }} > weekly/sha.txt
-
-      - name: Check SHA
-        id: sha
-        uses: actions/cache@v3
-        with:
-          path: weekly/sha.txt
-          key: weekly-sha-${{ github.sha }}
+          echo '::set-output name=run::'$(test -n "$(git log --format=%H --since=1 week ago')" && echo 'true' || echo 'false')
 
   distcheck:
     needs: schedule
-    if: ${{ needs.schedule.sha.outputs.cache-hit != 'true' }}
+    if: ${{ needs.schedule.check.outputs.run == 'true' }}
     runs-on: ubuntu-latest
     steps:
       - name: Install dependencies