Use UnionSourceAccessor to mount the chroot store on top of the real … #417

Job	Run time
Check Docker secrets present for installer tests	1s
eval	54s
vm_tests	24m 40s
tests on ubuntu	1m 57s
tests on macos	2m 37s
installer test ${{ matrix.scenario }}	0s
flake_regressions	14m 51s
docker_push_image	0s
	45m 0s

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