Introduce `libnixflake` #10761

Job	Run time
vm_tests	22m 29s
Check Cachix and Docker secrets present for installer tests	1s
meson_build (ubuntu-latest)	7m 19s
meson_build (macos-latest)	3m 14s
tests (ubuntu-latest)	32m 18s
tests (macos-latest)	33m 11s
installer	0s
docker_push_image	0s
installer_test	0s
	1h 38m 32s

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