Fixes artifact upload for platform cross building

[mdedetrich]

This PR is an upstream port of https://github.com/typelevel/sbt-typelevel/pull/98, every commit has been cherry picked both for authorship acknowledgement and also to make it easy to correlate the commits. In the process of cherry picking I have also done modifications so that the project compiles and the tests run (i.e. for each cherry picked commit I ran sbt githubWorkflowGenerate for both the root build and every sbt scripted test). Exceptions are noted below

• I have added the Handle case where githubWorkflowBuildMatrixAdditions is empty commit myself as this appears to be an accidental regression. In sbt-typelevel there doesn't seem to be a case where githubWorkflowBuildMatrixAdditions is empty but this is typically the case for standard trivial builds and the github workflow generation in this case appeared to be incorrect (i.e. trailing - and including rootJVM by itself when not necessary). Please check if this is intended, particularly the removal of the rootJVM part (I may be missing something here)
• I did not cherry pick the https://github.com/typelevel/sbt-typelevel/pull/98/commits/1801859216cca487d7fedb125ca08ecad9a8b55c commit as this appeared to be specific for typelevel-sbt and how they structure their projects. We should probably add a new commit that either modifies an existing test or makes a new one so we set githubWorkflowArtifactDownloadExtraKeys to some value to test that it works @djspiewak @armanbilge wdyt? Feel free to just add the commit onto this PR if you want to do it yourself (maintainers can add commits onto this PR).