chore: Set permissions for GitHub actions

[nathannaveen]

Restrict the GitHub token permissions only to the required ones; this way, even if the attackers will succeed in compromising your workflow, they won’t be able to do much.

• Included permissions for the action. https://github.com/ossf/scorecard/blob/main/docs/checks.md#token-permissions

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#permissions

https://docs.github.com/en/actions/using-jobs/assigning-permissions-to-jobs

Keeping your GitHub Actions and workflows secure Part 1: Preventing pwn requests

Signed-off-by: nathannaveen 42319948+nathannaveen@users.noreply.github.com

[gitpod-io[bot]]

[roed314]

Note that SageMath doesn't use github for pull requests; see the developer guide for more details.