Better pre-release CI workflow

[LucaMarconato]

• I'll modify the test.yaml workflow at a later point in a follow-up PR.

Originally posted by @LucaMarconato in #1034 (comment)

Move the pre-release job outside PR tests, and instead turn it into a periodic action run on main. Otherwise a PR may be blocked due to unrelated code changes (e.g. a pre-release in numpy breaking numba).