Allow skipping the "Upload artifact" step to allow usage in Forgejo Actions

[ensko]

This action is almost compatible with Forgejo actions, and very useful also there: it handles different runtimes and package managers, handles caching, and of course builds the Astro site. However, the last step uses actions/upload-artifact, which can only be used when running on Github.

To allow users to use the built site in other ways, by simply accessing the dist/ directory in another step of the same job, an option to skip the "Upload artifact" step would be great. The "Archive artifact" step builds artifact.tar, which could also be used by subsequent steps, so skipping Upload should probably not automatically skip Archive, although allowing to skip that would be an option. A possible interface would be to add an input result-kind with options dir | tar | upload (modulo bikeshedding) that decides up to which point the action is run.

If this feature is desirable and we figure out how it should look, I would gladly submit a PR to add the option.

[ensko]

After a tiny bit of testing (here, mirrored on Github here), it seems that even when not executing the Upload step, the build fails due to not finding the denoland/setup-deno action. So it seems the proposed change alone wouldn't make the action work on Forgejo, making this a bit less viable...