infra: reduce toil of manually sync workflows between contrib and core (generate-workflows-lib)

[emdneto]

Frequently we face situations when we change tox.ini in contrib (e.g., adding a new env or instrumentation or fixing a env name) we are required to manually update workflows in core as well (we need this to test core changes don't break contrib). Here are some PRs:

#4258
#4256
#4239
#4192
#4158

Maybe there's a better way to do that without the need to wait someone deal with a broken PR in core because tox -e generate-workflows is failing in CI.

Right now, the solution is: we need to manually fix workflows (i.e., run tox -e generate-workflows locally) and commit the changes (this is the process for keep workflows synced between -core and -contrib).

1. We need to have this explicit in documentation as well.
2. Investigate better way to automate this.

[xrmx]

I think this is similar to open-telemetry/opentelemetry-python-contrib#2965 and that a fix for one will fix the other too.