Link to build artifacts in PR comments

[benjamink]

This change to the autobuild.yml GitHub Action will upload the build artifacts for each of the firmware builds when changes are pushed to a PR & comment in the PR with links to download the latest artifacts. Subsequent pushes to the PR will rebuild the artifacts & the links in the persistent comment will be updated to point to the new artifacts. This workflow will only create a single comment in a PR with the links & just keep updating that one comment so it doesn't become noisy.

By default GitHub stores build artifacts for 90 days & then they are automatically deleted.

This is an example of how the comment should look:

[dillera]

This is a draft- do you want it merged- need to flip it to a real PR.

[benjamink]

Converting this to ready for review. I was tempted to tweak it more to add the short SHA to the artifact names but I think we should just merge this to see if it actually works first.

[FozzTexx]

Could you make a separate PR with just the autobuild.yml changes? I think that needs to go in before this one in order for the checks to pass. Don't close or change this one, just make another PR with just autobuild.yml.

[benjamink]

This is not going to work due to permissions. The only way to make this work is to enable sending write tokens with the GITHUB_TOKEN environment variable in jobs. This has to be done by a repo admin in the repo Settings & probably isn't what we want for a public repo. Here's details from the GitHub docs:

https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#changing-the-permissions-in-a-forked-repository