ci: add GitHub token permissions for workflow

[varunsh-coder]

This PR adds minimum token permissions for the GITHUB_TOKEN using https://github.com/step-security/secure-workflows.

GitHub recommends defining minimum GITHUB_TOKEN permissions for securing GitHub Actions workflows

• https://github.blog/changelog/2021-04-20-github-actions-control-permissions-for-github_token/
• https://docs.github.com/en/actions/security-guides/automatic-token-authentication#modifying-the-permissions-for-the-github_token
• The Open Source Security Foundation (OpenSSF) Scorecards treats not setting token permissions as a high-risk issue

This project is part of the top 100 critical projects as per OpenSSF (https://github.com/ossf/wg-securing-critical-projects), so fixing the token permissions to improve security.

Signed-off-by: Varun Sharma varunsh@stepsecurity.io

[cowtowncoder]

Sounds good. I'll let CI run to verify things still work as I'd expect (there's no need for write access by anything here) and can then merge.

[cowtowncoder]

Thank you @varunsh-coder!