My guess is that a 2015 Macbook Air is probably not going to run a MacOS version that is still supported by Apple. That would be yet another reason to simply install Linux. Before you do so you can go for https://rescuezilla.com/ and do disk cloning to an image that you save to some storage like a USB disk. If you do the same after your installing and tweaked Linux installation, you can have the best of both worlds whenever you need it.
Is this where the source code is supposed to be ? https://github.com/ProtonMail/inbox-desktop