chore: ci: request contents read permissions explicitly in gha

[galargh]

Related Issues

n/a

Proposed Changes

Request contents read permissions explicitly in GitHub Actions.

Additional Info

This permissions are granted implicitly in public repositories (like lotus), but need to be requested explicitly in private repositories (like lotus-priv). This change will allow us to finish the migration of lotus-priv to GHA.

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• If the PR affects users (e.g., new feature, bug fix, system requirements change), update the CHANGELOG.md and add details to the UNRELEASED section.
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green

[github-actions]

All checks have passed