implement SailfishOS support, made in collaboration with vakkov

sorry for building your hopes up, it currently segfaults:
https://bpaste.net/show/319c346897f6
i hope someone can help me out there

i've continued what vakkov started with cleanups and i tried to make it so that other platforms still work (but i cannot test all of them :()
ah yes, forgot about that, install_sailfish is already a phony target: https://github.com/thp/apkenv/pull/55#discussion-diff-21745480R16