ci.yml: Downgrade to free x86_64-darwin runner #11568

Job	Run time
Check Cachix and Docker secrets present for installer tests	4s
vm_tests	22m 36s
meson_build (ubuntu-latest)	10m 16s
meson_build (macos-latest, aarch64-darwin)	4m 45s
meson_build (macos-13, x86_64-darwin)	8m 12s
tests (ubuntu-latest)	50m 57s
tests (macos-latest, aarch64-darwin)	35m 34s
tests (macos-13, x86_64-darwin)	12m 46s
flake_regressions	29m 16s
installer	0s
docker_push_image	0s
installer_test (${{ matrix.os.name }})	0s
	2h 54m 26s

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