CI: run rubocop only once

[bastelfreak]

previously, we called the default rake task, which runs the unit tests and afterwards rubocop. That doesn't make much sense. We usually run rubocop once, before generating the matrix. We can do the same here.

Short description

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)