ci: add minimum GitHub token permissions for workflows

[ashishkurmi]

Description

This PR adds minimum token permissions for the GITHUB_TOKEN in GitHub Actions workflows using secure-workflows.

The GitHub Actions workflow has a GITHUB_TOKEN with write access to multiple scopes. Here is an example of the permissions in one of the workflow runs:
https://github.com/stan-dev/math/actions/runs/3153019054/jobs/5128975789#step:1:19

After this change, the scopes will be reduced to the minimum needed for the following workflows:

• header_checks.yml
• main.yml

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.
• 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: Ashish Kurmi [email protected]

[WardBrian]

Looks good! Thanks for bringing this up