.mergify: Use tianocore-issues to rebase PRs for merge

Periodically, mergify rebase operations will fail because
an account is selected to perform the rebase that does
not have permissions to do the rebase.

Insetad, use the tianocore-issues account to perform the
rebase operation.

Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>

.mergify/config.yml

@@ -30,6 +30,7 @@ queue_rules:
       - base~=(^main|^master|^stable/)
       - label=push
     merge_method: rebase
+    update_bot_account: tianocore-issues
 
 pull_request_rules:
   - name: Automatically merge a PR when all required checks pass and 'push' label is present