MDBF-801: Add github status link to full Buildbot results

[grooverdan]

• The Jira issue number for this PR is: MDBF-801

Description

This enable users that submit pull requests to be able to easily see the link to the full test results of BuildBot.

As @ParadoxV5 commented when prompted about the mistake #4200 fixed, "why do no S390x builders advertise themselves to PRs?". With this, we can make it obvious rather than having users know /remember specific URL patterns.

Release Notes

nothing

How can this PR be tested?

See if the next PR gets a friendly GH comment.

Concept tested by @RazvanLiviuVarzaru
RazvanLiviuVarzaru/buildbot-r#7

If the changes are not amenable to automated testing, please explain why not and carefully describe how to test manually.

Basing the PR against the correct MariaDB version

• This is a new feature or a refactoring, and the PR is based against the main branch.
• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

[ParadoxV5]

GitHub Action, woah

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I’m not fond of letting a bot take over every 1st floor after OP, though, be it a spell checker or even @CLAassistant.
Because GitHub has a dedicated section for those: Checks, where Appveyor and the reported builders are.

[grooverdan]

back to MariaDB/buildbot#743 perhaps

[vuvova]

I agree with @ParadoxV5 here. That is, I don't mind when a bot takes over every 1st floor after OP, if needed. But there's a dedicated section in the PR for CI links, and comments aren't where everyone expects a CI link.