Fix unneeded recipe workflow

To work for PRs created from forked repos.

Fixes: #329

.github/workflows/unneeded_recipe_updates.yml

@@ -1,29 +1,40 @@
 name: Check for unneeded recipe updates
 
 on:
-  pull_request:
+  pull_request_target:
     paths:
       - 'docs/recipes/**/*.md'
 
 jobs:
   check_updates:
     runs-on: ubuntu-latest
 
-    steps:          
+    steps:
+      # Checkout the base repository (not the fork) to avoid running untrusted code
+      - name: Checkout base repository
+        uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.base.ref }}
+
       - name: Add warning comment to pull request
         uses: actions/github-script@v6
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           script: |
+            const prNumber = context.payload.pull_request?.number;
+            if (!prNumber) {
+              throw new Error('No pull request found.');
+            }
+            
             const output = `
             While we appreciate that you're attempting to improve the docs, please be aware that all files in the \`/docs/recipes\` directory are auto-generated by the [markdown-generator](https://github.com/openrewrite/rewrite-recipe-markdown-generator).
 
             Please make changes to the recipes themselves rather than to these files. When the next release happens, these files will be updated accordingly.
-            `
+            `;
 
-            github.rest.issues.createComment({
-              issue_number: context.issue.number,
+            await github.rest.issues.createComment({
+              issue_number: prNumber,
               owner: context.repo.owner,
               repo: context.repo.repo,
               body: output
-            })
+            });