Fix GitHub rate limiting when generating pull request descriptions

[rhyskoedijk]

Refs #1356.

Dependabot hits api.github.com to fetch project info, release notes, and commit messages for dependencies when generating pull request descriptions. Currently these requests are unauthenticated because we do not set the GitHub access token in the job credentials, which leads to errors like:

2024-09-27T18:37:21.5657248Z   proxy | 2024/09/27 18:37:21 [952] Remote response: {"message":"API rate limit exceeded for xxx.xx.xx.x. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)","documentation_url":"https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting"}
2024-09-27T18:37:21.5695270Z updater | 2024/09/27 18:37:21 ERROR <job_update_0_npm_and_yarn_all> Error while generating PR message: GET https://api.github.com/repos/angular/angular/contents/packages/language-service: 403 - API rate limit exceeded for xxx.xx.xx.x . (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.) // See: https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting
2024-09-27T18:37:21.5697807Z updater | 2024/09/27 18:37:21 ERROR <job_update_0_npm_and_yarn_all> /home/dependabot/dependabot-updater/vendor/ruby/3.3.0/gems/octokit-7.2.0/lib/octokit/response/raise_error.rb:14:in `on_complete'
2024-09-27T18:37:21.5698373Z updater | /home/dependabot/dependabot-updater/vendor/ruby/3.3.0/gems/faraday-2.7.11/lib/faraday/middleware.rb:18:in `block in call'

This change adds a credential entry for github.com if the GitHub access token task input is set.

This change also forwards the Azure DevOps access token to dependabot via the LOCAL_AZURE_ACCESS_TOKEN environment variable, which is used here. It doesn't currently change any behaviour since the token is already supplied explicitly in the job credentials; Just added for consistency.