Ability to customize checkout behavior

**Is your feature request related to a problem? Please describe.**
I want to have a pipeline that triggers on a pull request, but checks out the main branch and runs some scripts from there to decide whether it's ok to run builds on that PR. Currently I'm using a [skip-checkout](https://github.com/thedyrt/skip-checkout-buildkite-plugin) plugin and then `wget` to manually fetch all the files. It's a bit gross. I could do a manual `git fetch` instead of `wget` here, but that's pretty verbose and copy-pasta from the Buildkite checkout logs. There's also the [smooth-checkout](https://github.com/hasura/smooth-checkout-buildkite-plugin) plugin, which unfortunately also has some [custom behavior for checkout directory](https://github.com/hasura/smooth-checkout-buildkite-plugin/issues/13). I think there are a fair number of plugins and feature requests around more control of the checkout process. I could of course create my own hook, but I find agent hooks a really unpalatable mechanism for this sort of configurability because they're totally opaque to the pipeline. They make sense for things like locking down agent security, but not really for customizing individual step behavior.

Another use case is checking out the merge commit for a PR instead of the head commit, which there are a few plugins implementing in various ways, all of which involve somewhat complex configuration over multiple steps:
- https://github.com/seek-oss/github-merged-pr-buildkite-plugin
- https://github.com/juliaci/pull-request-merge-commit-buildkite-plugin

**Describe the solution you'd like**
The commit buildkite checks out after fetching should be something settable with an environment variable. Then I could set `BUILDKITE_REFSPEC=refs/heads/main` and `BUILDKITE_CHECKOUT_COMMIT=FETCH_HEAD` using something like the [metahook plugin](https://github.com/improbable-eng/metahook-buildkite-plugin). I think it's good to leave `BUILDKITE_COMMIT` immutable and default `BUILDKITE_CHECKOUT_COMMIT` to `BUILDKITE_COMMIT`.

**Describe alternatives you've considered**
An option in Buildkite to always checkout `FETCH_HEAD`

Ideally, this would be something that Buildkite would offer more direct support for, but I think the method using environment variables and leveraging this existing plugin would be easier in the short term.

I could also write my own plugin or contribute to one of the existing checkout customization plugins, but based on my survey of them it seems like they frequently end up being especially tailored to an individual use case and also reinventing a lot of Buildkite's checkout logic. It's pretty heavy-weight to have to create an entire repository to check out a different commit.

Some other plugins customizing checkout behavior:
- https://github.com/arromer/github-fetch-buildkite-plugin
- https://github.com/uber-workflow/run-without-clone-buildkite-plugin
- https://github.com/first-aml/git-clone-buildkite-plugin
- https://github.com/peakon/git-shallow-clone-buildkite-plugin
- https://github.com/pragmaplatform/sparse-checkout-buildkite-plugin

No files, tests, or entry points are named in the issue. Start by locating the agent's checkout implementation and its environment-variable handling, then review how checkout refspecs and commits are currently selected. Done means supporting the requested configurable checkout behavior with tests for the default and customized cases.

Written by the indexing model from the issue text.