Explicitly disable sparse checkout unless asked for

[dscho]

When a worktree is reused by actions/checkout and the first time sparse checkout was enabled, we need to ensure that the second time it is only a sparse checkout if explicitly asked for. Otherwise we need to disable the sparse checkout so that a full checkout is the outcome of this Action.

This fixes #1475

[dscho]

It looks as if the test-proxy job fails even without these here changes, so I'll call this PR good to go.

[jww3]

do you want to pop the .startGroup / .endGroup calls outside of the if block?

Consider core.startGroup('Applying sparse checkout options');

[dscho]

No, because we're not actually setting up a sparse checkout. We're disabling it. But saying that would only make sense if there was a worktree already, otherwise there'd be no sparse checkout to disable.

[joshmgross]

We should be very careful about adding in external dependencies to our workflows - https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions

If this is something we internally built, could we publish it to ghcr.io within this repository or org?

[jww3]

Agreed. This was something @fhammerl provided as a proof-of-concept, but I was just reading up about how to publish packages to ghcr.io and like the idea of hosting this container image at ghcr.io/actions (see neighboring packages here)