[feat req] merging of uploads ala v3

[jadamcrain]

What would you like to be added?

V3 allowed you to merge artifacts from multiple jobs. V4 removed this capability:

Uploading to the same named Artifact multiple times.

Due to how Artifacts are created in this new version, it is no longer possible to upload to the same named Artifact multiple times. You must either split the uploads into multiple Artifacts with different names, or only upload once. Otherwise you will encounter an error.

This will break a lot of parallelized pipelines. For example, our pipeline builds native libraries for a number of architectures in parallel in a matrix:

https://github.com/stepfunc/dnp3/blob/1b25ad1a6ef09729d205d36d20b2168d1d4f75f1/.github/workflows/ci.yml#L199

We then upload all of the built artifacts to a common artifact name for each architecture:

https://github.com/stepfunc/dnp3/blob/1b25ad1a6ef09729d205d36d20b2168d1d4f75f1/.github/workflows/ci.yml#L241

V4 completely breaks this way of doing things without providing a work around or suggested alternative pattern.

Why is this needed?

It's a common pattern that existed in V3 and will now break many many builds in V4

[SteffenL]

I faced a similar issue but the migration guide provided the solution.

[jadamcrain]

Thanks @SteffenL, I will have a look. I missed the migration guide in the release notes.

[kenorb]

See: https://github.com/actions/upload-artifact/tree/main/merge#combining-all-artifacts-in-a-workflow-run

Related: GH-520, GH-524.