Correctly set the source repository when doing PR validation (#328)

### Description

The previous workflow was just using the current repositories, which
worked when I was testing but didn't work for actual PRs.
 
### Issues Resolved

No Issue

### Check List
- [X] Commits are signed per the DCO using `--signoff`

By submitting this pull request, I confirm that my contribution is made
under the terms of the BSD-3-Clause License.

Signed-off-by: Madelyn Olson <madelyneolson@gmail.com>

Author: madolson

.github/workflows/zola-deploy.yml

@@ -24,6 +24,9 @@ jobs:
           ref: >-
             ${{ github.event_name == 'pull_request' &&
                 github.head_ref || 'main' }}
+          repository: >-
+            ${{ github.event_name == 'pull_request' &&
+                github.event.pull_request.head.repo.full_name || 'main' }}
           path: website
 
       - name: Checkout valkey-doc