ci: add GitHub token permissions for workflow (facebook#10549)

Summary:
This PR adds minimum token permissions for the GITHUB_TOKEN in GitHub Actions workflows 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](https://github.com/ossf/scorecard) 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.

Before the change:
`GITHUB_TOKEN` has `write` permissions for multiple scopes, e.g.
https://github.com/facebook/rocksdb/runs/7936368166?check_suite_focus=true#step:1:19

After the change:
`GITHUB_TOKEN` will have minimum permissions needed for the jobs.

Signed-off-by: Varun Sharma <[email protected]>

Pull Request resolved: facebook#10549

Reviewed By: ajkr

Differential Revision: D38923184

Pulled By: jay-zhuang

fbshipit-source-id: 0c48f98fe90665e53724f57a7d3b01dd80f34a93

Author: varunsh-coder

.github/workflows/sanity_check.yml

@@ -1,5 +1,8 @@
 name: Check buck targets and code format
 on: [push, pull_request]
+permissions:
+  contents: read
+
 jobs:
   check:
     name: Check TARGETS file and code format