Make the external help integration optional

The external help repository is private due to license requirements.
This change ensures that PRs created by external contributors can still generate the pack as a validation step, but
without the external help files.

Author: rvermeulen

.github/workflows/code-scanning-pack-gen.yml

@@ -184,6 +184,8 @@ jobs:
           add-to-path: false
 
       - name: Checkout external help files
+        continue-on-error: true
+        id: checkout-external-help-files
         uses: actions/checkout@v2
         with:
           ssh-key: ${{ secrets.CODEQL_CODING_STANDARDS_HELP_KEY }}
@@ -192,6 +194,7 @@ jobs:
           path: external-help-files
 
       - name: Include external help files
+        if: ${{ steps.checkout-external-help-files.outcome == 'success' }}
         run: |
           pushd external-help-files
           find . -name '*.md' -exec rsync -av --relative {} "$GITHUB_WORKSPACE" \;