fix: use base SHA instead of branch name in base spec URL

Author: reuvenharrison

.github/workflows/pr-comment.yml

@@ -18,6 +18,6 @@ jobs:
     - name: Post oasdiff PR comment
       uses: oasdiff/oasdiff-action/pr-comment@v0.0.34
       with:
-        base: https://raw.githubusercontent.com/${{ github.repository }}/${{ github.base_ref }}/simple.yaml
+        base: https://raw.githubusercontent.com/${{ github.repository }}/${{ github.event.pull_request.base.sha }}/simple.yaml
         revision: simple.yaml
         oasdiff-token: 0634345d-02fb-43df-b56a-68fc22253621