Upload artifact fails if target directory doesn't exist

[mpilquist]

Ran in to this on fs2, where the build action runs test in certain sub-projects only. Hence, some subprojects don't have a target directory and the upload fails.

https://github.com/functional-streams-for-scala/fs2/actions/runs/137190325

[djspiewak]

Unfortunately, there's no way in general for this plugin to tell what target directories will or will not get generated, and there are no file existence predicates available in GitHub Actions. However, you can achieve this in the following way: https://github.com/djspiewak/sbt-github-actions/blob/3c40ebb75d0e3f170cb4e72417ef59effa12ef27/src/sbt-test/sbtghactions/non-existent-target/build.sbt#L10-L19

It's a little annoying, but it works and you really only have to do it once.