Fix CI job to check for signed commits

The previously version of the check-signed-commits-action did not
support forks. So bumping to the latest version which does support
forks.

I've switched to @main so we automatically keep up with new updates
to the check-signed-commits-action without action in this repo
required.

Author: SimonBarendse

.github/workflows/pr-check-signed-commits.yml

@@ -13,4 +13,4 @@ jobs:
         uses: actions/checkout@v3
 
       - name: Check signed commits in PR
-        uses: 1Password/check-signed-commits-action@v1
+        uses: 1Password/check-signed-commits-action@main