CI: Extract and don't hardcode artifact naming

[cprecioso]

Description

• As part of CI: Generate npm packages #3464

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• I extracted the computation of the artifact and tarball name out of the waspc-build workflow, so that we can reuse it in multiple places (mainly, the fetch-nightly-packages; and in the future the npm packaging script) without repeating ourselves.
• Makes the waspc-build job output the computed names of the artifacts it has created. This way we don't hardcode these names in the following workflows.
  • This requires a bit of a workaround since, in matrix jobs, each job will override the output. Instead, we store each job's output in a different variable.

Type of change

• 🔧 Just code/docs improvement
• 🐞 Bug fix
• 🚀 New/improved feature
• 💥 Breaking change

Checklist

• I tested my change in a Wasp app to verify that it works as intended.

• 🧪 Tests and apps:

  • I added unit tests for my change.
  • (if you fixed a bug) I added a regression test for the bug I fixed.
  • (if you added/updated a feature) I added/updated e2e tests in examples/kitchen-sink/e2e-tests.
  • (if you added/updated a feature) I updated the starter templates in waspc/data/Cli/templates, as needed.
  • (if you added/updated a feature) I updated the example apps in examples/, as needed.
    • (if you updated examples/tutorials) I updated the tutorial in the docs (and vice versa).

• 📜 Documentation:

  • (if you added/updated a feature) I added/updated the documentation in web/docs/.

• 🆕 Changelog: (if change is more than just code/docs improvement)

  • I updated waspc/ChangeLog.md with a user-friendly description of the change.
  • (if you did a breaking change) I added a step to the current migration guide in web/docs/migration-guides/.
  • I bumped the version in waspc/waspc.cabal to reflect the changes I introduced.

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