mirrors: Set permissions in mirror repo workflows

[anomiex]

Closes MONOREP-284

Proposed changes:

Set permissions in mirror repo workflows

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

None

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• You'd have to clone a mirror repo using these, update it, and then see if they still run.

[github-actions]

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