ci: add outputs to workflow_call on generic_build

[georglauterbach]

Description

Looking at https://docs.github.com/en/actions/using-workflows/reusing-workflows#using-outputs-from-a-reusable-workflow, we can see that we need to manually set the outputs for a whole generic workflow. This commit fixes an issue where the cache key was not set properly as the input was empty (because the output of the previous job was not actually set).

For reference, see #2772 (comment)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes