Allow default GH token to write content in the build script

[yubiuser]

What does this PR aim to accomplish?:

We use https://github.com/softprops/action-gh-release to attach files to the release. They always needed content: write permission. So far this was not an issue, but the scope of the default GH token seems to have changed (softprops/action-gh-release#572 (comment)).
This PR grants the necessary permissions.

See a failing action trigged by a release tag here: https://github.com/pi-hole/FTL/actions/runs/13396405826

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[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/v6-post-release-fixes-and-findings/76480/1