exec: fix incorrect deps computation for special mounts #4503

Job	Run time
govulncheck	53s
prepare	4s
binaries (darwin/amd64)	58s
binaries (darwin/arm64)	1m 0s
binaries (linux/amd64)	1m 54s
binaries (linux/arm/v7)	1m 51s
binaries (linux/arm64)	1m 52s
binaries (linux/s390x)	1m 54s
binaries (linux/ppc64le)	1m 54s
binaries (linux/riscv64)	2m 10s
binaries (windows/amd64)	1m 38s
binaries (windows/arm64)	1m 38s
test / prepare	2m 11s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 46s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	8m 57s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 17s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	8m 45s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 22s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	4m 57s
test / run (oci-rootless-slirp4netns-detachnetns, ./client ./cmd/buildctl ./worker/containerd ./solver ....	5m 7s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	5m 1s
test / run (./..., 1, integration gateway)	4m 26s
test / run (./client, containerd, nydus, integration)	7m 33s
test / run (./client, oci, nydus, integration)	5m 9s
test / run (./..., nydus, 1, integration)	4m 28s
image	8m 30s
image (rootless)	8m 44s
release	10s
scout	0s
	1h 52m 9s

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