actions: use GitHub PAT with project write permissions

Previously, the default token had no access to write to projects.

This PR changes the action to use the token with proper permissions set.

Release note: None

.github/workflows/add-mt-issues-to-project.yml

@@ -14,6 +14,6 @@ jobs:
           # URL of the project to add issues to
           project-url: https://github.com/orgs/cockroachdb/projects/36/
           # A GitHub personal access token with write access to the project
-          github-token: ${{ secrets.GITHUB_TOKEN }}
+          github-token: ${{ secrets.ADD_TO_PROJECT_PAT }}
           # A comma-separated list of labels to use as a filter for issue to be added
           labeled: T-multitenant