ci: fix missing permission for the stale workflow (#15342)

Permissions were restricted (following documentation of actions/stale)
in 51ad150d9267fd22caa03dc5ddde5b15bb52c5cfd017217da30270b5c10ab6ae, but
the `actions: write` is missing

This permission is only mentioned in the top of documentation along with
many other "optional" permissions that depend upon which features you
use... but it is needed for the action to manipulate the actions cache
the way that it does.

When operations limit is exceeded, it "stops" and "picks up from where
it left off" in the next run, and actions cache is used to do this.

Author: rmuir

.github/workflows/mark-stale-PRs.yml

@@ -19,6 +19,7 @@ jobs:
 
     permissions:
       pull-requests: write # must comment/label PRs
+      actions: write # must manipulate actions cache to split workload across multiple runs
 
     steps:
       - name: Run stale PR action