Grant comment workflow permissions

[willcl-ark]

Only require the minimal permissions to write a comment on a pull-request.

Allows repo-level workflow permissions to be set to read-only:

Read repository contents and packages permissions
Workflows have read permissions in the repository for the contents and packages scopes only.

[0xB10C]

ACK 065245b

From the docs for create an issue comment:

The fine-grained token must have at least one of the following permission sets:

• "Issues" repository permissions (write)
• "Pull requests" repository permissions (write)

The actions: read permissions don't seem to be required (or might already be granted) as can be seen in e.g. this runs download artifact step: https://github.com/bitcoin-core/guix.sigs/actions/runs/9579230000/job/26411233646#step:2:19