Add minimum GitHub token permissions for workflow

[varunsh-coder]

Description

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

GitHub Actions workflows have a GITHUB_TOKEN with write access to multiple scopes.
Here is an example of the permissions in one of the workflows:
https://github.com/halide/Halide/actions/runs/3024567888/jobs/4866106064#step:1:19

After this change, the scopes will be reduced to the minimum needed for the workflow.

Motivation and Context

• This is a security best practice, so if the GITHUB_TOKEN is compromised due to a vulnerability or compromised Action, the damage will be reduced.
• GitHub recommends defining minimum GITHUB_TOKEN permissions.
  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 also treats not setting token permissions as a high-risk issue. This change will help increase the Scorecard score for this repository.

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

[alexreinking]

Thanks for the patch!