BUG: Add permissions to apply clang format action

[thewtex]

Attempt to enable the pull request clang format action to work on forked pull
requests.

Background:

• starter-workflow template gives Resource not accessible by integration actions/labeler#12
• https://github.blog/changelog/2021-04-20-github-actions-control-permissions-for-github_token/

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any)
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• Added ITK examples for all new major features (if any)

Refer to the ITK Software Guide for
further development details if necessary.