This pull request updates GitHub Actions workflow files to explicitly
set permissions for certain actions. (fixing the permissions issues)
**Workflow permissions updates:**
* Added explicit `issues: write` permission to the `assign.yml` workflow
to ensure it can modify issues as needed.
* Set `contents: write`, `actions: write`, and `checks: write`
permissions in the `build_all.yml` workflow to allow the workflow to
interact with repository contents, manage actions, and update checks.
Allow users to assign issues to themselves.
I don't know if its comments (i.e. unassigning) affects orca bot's
inactivity and stale. Probably. I didn't see a way for actions/stale
to exempt particular comments. This is set for 30 days and stale for
90 days, so if people are using this the stale timing can jump up to
120 days.
