workflows: Use correct access token when pushing to llvm-project-release-prs repo

The checkout action will hard-code the default github actions token in
the git config so that all pushes use it.  We need to set
persist-credentials=false so we can use a token that has permission
to push to the llvm-project-release-prs repo.

.github/workflows/issue-release-workflow.yml

@@ -72,6 +72,8 @@ jobs:
     steps:
       - name: Fetch LLVM sources
         uses: actions/checkout@v2
+        with:
+          persist-credentials: false
 
       - name: Setup Environment
         run: |