Unify workflow yamls with pre-processing step

[padelsbach]

• Use a new script to pre-process yamls and support include: directive to pull in shared code
• Generated files are checked in, and pre-processing ensured by new (non-generated) yaml
• Removes about 70 lines of common code and boilerplate setup

Notes on this approach:
Pros:

• simple yaml-like syntax, still looks like workflow yaml
• shell script based, no additional installs (jinja2 was another option considered)

Cons:

• re-generation check is only done in CI
• include is only feature, no support for function-like macros or passing args