Fix mountpoints for nix store paths that are files #561

Job	Run time
build (nix-snapshotter)	22s
build (nix-snapshotter)	22s
build (nix-snapshotter)	22s
lint (golangci-lint)	1m 0s
lint (golangci-lint)	1m 0s
lint (golangci-lint)	1m 0s
build (image-hello)	20s
build (image-hello)	20s
build (image-hello)	20s
build (image-redis)	21s
build (image-redis)	21s
build (image-redis)	21s
build (image-redisWithShell)	23s
build (image-redisWithShell)	23s
build (image-redisWithShell)	23s
integration-test (snapshotter)	2m 8s
integration-test (snapshotter)	2m 8s
integration-test (snapshotter)	2m 8s
integration-test (push-n-pull)	1m 28s
integration-test (push-n-pull)	1m 28s
integration-test (push-n-pull)	1m 28s
integration-test (kubernetes)	19m 15s
integration-test (kubernetes)	19m 22s
integration-test (kubernetes)	19m 24s
integration-test (k3s)	12m 49s
integration-test (k3s)	12m 49s
integration-test (k3s)	12m 49s
integration-test (k3s-external)	19m 59s
integration-test (k3s-external)	19m 59s
integration-test (k3s-external)	18m 37s
integration-test (k3s-rootless)	13m 8s
integration-test (k3s-rootless)	13m 8s
integration-test (k3s-rootless)	13m 8s
integration-test (gvisor)	1m 28s
integration-test (gvisor)	1m 28s
integration-test (gvisor)	1m 28s
coverage-report	0s
coverage-report	0s
coverage-report	0s
	3h 36m 57s

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