Move plugins infra to `libnixmain` #11177

Job	Run time
Check Cachix and Docker secrets present for installer tests	1s
meson_build (ubuntu-latest)	10m 9s
meson_build (macos-latest)	4m 45s
vm_tests	21m 47s
tests (ubuntu-latest)	51m 20s
tests (macos-latest)	43m 24s
docker_push_image	0s
installer	0s
installer_test	0s
	2h 11m 26s

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