Move plugins infra to `libnixmain` #11175

Job	Run time
vm_tests	21m 38s
Check Cachix and Docker secrets present for installer tests	1s
meson_build (ubuntu-latest)	6m 48s
meson_build (macos-latest)	3m 43s
tests (ubuntu-latest)	47m 58s
tests (macos-latest)	36m 39s
installer	0s
docker_push_image	0s
installer_test	0s
	1h 56m 47s

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