Build branches other than 'main'

[tomjelinek]

This change allows forks to keep 'main' as the branch that follows upstream, while specific courses have their own branches.

[tomjelinek]

@encukou @hroncok Ahoj, jak bude čas, mrkni prosím, jestli dává smysl tohle mergnout do mainu. Díky.

[encukou]

Let's do this. It'll also need a change in the naucse.python.cz definitions, later.

Thank you for the PR!