Fix the signing commits check GitHub workflow for PRs from forks

I also upgraded the actions/checkout GitHub Action and made the names clearer.

Author: fharper

.github/workflows/check-signed.yml

@@ -1,15 +1,14 @@
 ---
-name: Check signed commits
-on: pull_request
+name: Validate if commits are signed
+on: [pull_request, pull_request_target]
 
 jobs:
   signed-commits-check:
-    name: Validate if commits in PR are signed
     runs-on: ubuntu-latest
     steps:
 
-      - name: Check out code
-        uses: actions/checkout@v3
+      - name: Check out the repository code
+        uses: actions/checkout@v4.1.4
 
-      - name: Check signed commits in PR
+      - name: Check if the commits are signed
         uses: 1Password/check-signed-commits-action@v1