Fix github action "performance-8.0" when PR is from a fork.

There is an assumption that the PR is from the same remote as
the base it is being merged into. This is fixed so that instead
of doing a `git fetch origin`, it now fetches from the `clone_url`
of the PR head.

Signed-off-by: Peter Serwylo <peter@serwylo.com>

.github/workflows/performance.yml

@@ -51,8 +51,9 @@ jobs:
 
       - name: Apply PR
         run: |
-          git fetch origin ${{ github.event.pull_request.head.ref }}
-          git checkout ${{ github.event.pull_request.head.ref }}
+          git remote add pr ${{ github.event.pull_request.head.repo.clone_url }}
+          git fetch pr ${{ github.event.pull_request.head.ref }}
+          git checkout -b pr/${{ github.event.pull_request.head.ref }}
           git submodule update
 
           ./occ upgrade