fix: use RELEASE_PLZ_TOKEN to trigger workflows on release PRs (#131)

## Summary
- Replace GITHUB_TOKEN with RELEASE_PLZ_TOKEN in release-plz workflow to
enable CI workflows to run automatically on release PRs

## Problem
When GitHub Actions creates a PR using the default `GITHUB_TOKEN`,
GitHub intentionally doesn't trigger other workflows to prevent infinite
loops. This meant our release PRs (#117) didn't run CI checks
automatically.

## Solution
Use the `RELEASE_PLZ_TOKEN` (Personal Access Token) that has the
necessary `repo` and `workflow` permissions to create PRs that trigger
CI workflows normally.

## Changes
- Updated `.github/workflows/release-plz.yml` to use `${{
secrets.RELEASE_PLZ_TOKEN }}` instead of `${{ secrets.GITHUB_TOKEN }}`
in both jobs

## Testing
- The RELEASE_PLZ_TOKEN has already been created and added to repository
secrets
- Future release PRs will now automatically trigger CI workflows
- This change will be validated when the next release PR is created

Closes #130

Author: jwilger

.github/workflows/release-plz.yml

@@ -30,7 +30,7 @@ jobs:
         with:
           command: release
         env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.RELEASE_PLZ_TOKEN }}
           CARGO_REGISTRY_TOKEN: ${{ secrets.CARGO_REGISTRY_TOKEN }}
 
   # Create a PR with the new versions and changelog, preparing the next release.
@@ -56,5 +56,5 @@ jobs:
         with:
           command: release-pr
         env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.RELEASE_PLZ_TOKEN }}
           CARGO_REGISTRY_TOKEN: ${{ secrets.CARGO_REGISTRY_TOKEN }}