Add BuildKite build agent

[aidenhaak]

Description

Added a Buildkite build agent so that GitVersion can use the BUILDKITE_BRANCH environment variable when running on a Buildkite agent.

Related Issue

Resolves #2956.

Motivation and Context

By default Buildkite checkout a branch in a headless state. This change removes the need for adding this at the start of a Buildkite pipeline:

git checkout $BUILDKITE_BRANCH
git reset HEAD --hard $BUILDKITE_COMMIT

How Has This Been Tested?

• Added unit tests
• Manually tested in a Buildkite pipeline

Please note I wasn't able to check my added documentation is valid as I got this error trying to run the build script:

❯ .\build.ps1 -Target Preview-Documentation
Running build stage build...

Tool Codecov.Tool is already installed, with required version.
Tool GitVersion.Tool is already installed, with required version.
Error: The target 'Preview-Documentation' was not found.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[aidenhaak]

Not sure what's going on with the documentation part of the build...any ideas @asbjornu ?

[asbjornu]

@aidenhaak, good question. There's nothing in this PR that should cause the "Could not find module remark-frontmatter" failure. Ideas @arturcic?

[arturcic]

@asbjornu it seems it's the [Remark Linting] step in the docs that are failing. If I remember correctly that was added by you some time ago as part of the docs validation. Can you check that?

[asbjornu]

Yeah, I noticed that. I just can't think of a reason for it to suddenly stop working. I'll have a look.

[asbjornu]

@aidenhaak, the build is now successful, but there are a few nitpicks pointed out by Reviewdog that would be great if you could fix. 🙏🏼

[aidenhaak]

@asbjornu fixed! Worth squashing 1a2bd5a and 925ff8c together?

[asbjornu]

@aidenhaak, awesome! No squashing needed. :)

[mergify]

Thank you @aidenhaak for your contribution!