Move GitHub actions to Linux

[LiarPrincess]

According to About billing for GitHub Actions the macOS minute costs 10 units. Given that a single build + tests take 6 min this is 60 units for a single check. This gives us 33 runs per month.

With PyTests (tests written in Python) + PyTests in release mode we will easily push above 10 min (100 units per run -> 20 runs per month).

On linux single minute costs 1 unit -> 333 runs (or 200 with PyTests).

Additional benefits:

• no Xcode on linux -> no tests for Ariel, but also no errors when Xcode version changes (this already happened in the past)
• I sometimes forget to check linux builds, this will automate the whole thingy

[youknowone]

Running on Linux may take less time, but you don't need to worry about billing unless this repository is public. The billing policy is adapted only for private repositories.