Git mirror isn't used

[brentleyjones]

Here the plugin tries to use the git mirror:

https://github.com/buildkite-plugins/sparse-checkout-buildkite-plugin/blob/a3eac8c55c181ce0ce2d17c57f932dc73e97179c/hooks/checkout#L39C48-L39C69

But since this plugin replaces the checkout hook, BUILDKITE_REPO_MIRROR isn't set.

This is inefficient since it doesn't use the git mirror, and if later we need to unshallow a branch, it will have to do a full fetch instead of utilizing the mirror.

[brentleyjones]

Though, after testing with BUILDKITE_REPO_MIRROR set to an already existing mirror, it was a lot slower.