Do not include a cache mount's ID in the ExecOp's cachemap #2233

Job	Run time
prepare	1s
test / prepare	1m 52s
test / run (containerd, ./frontend/dockerfile, integration)	4m 57s
test / run (containerd, ./frontend/dockerfile, dockerfile)	6m 14s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 34s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	5m 37s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 42s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	5m 49s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 30s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 31s
test / run (oci, ./frontend/dockerfile, integration)	3m 59s
test / run (oci, ./frontend/dockerfile, dockerfile)	5m 10s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 47s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 2s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	3m 58s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 5s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 37s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 40s
image (mainline)	5m 2s
image (labs)	4m 59s
release	0s
	1h 34m 6s

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