ci: do_not_merge: Use Ubuntu 20.04 runner image

This commit updates the "Do Not Merge" workflow to use a specific
runner image version, ubuntu-20.04, instead of the latest version in
order to prevent any potential breakages due to the 'latest' version
change by GitHub.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/do_not_merge.yml

@@ -8,7 +8,7 @@ jobs:
   do-not-merge:
     if: ${{ contains(github.event.*.labels.*.name, 'DNM') }}
     name: Prevent Merging
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Check for label
         run: |