ci: ignore error if gha cache export fails #5209

Job	Run time
prepare	1s
test / prepare	25m 45s
test / run (containerd, ./frontend/dockerfile, integration)	27m 17s
test / run (containerd, ./frontend/dockerfile, dockerfile)	26m 18s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	20m 36s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	27m 34s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	15m 34s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	27m 41s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	15m 34s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	22m 39s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	15m 37s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	26m 22s
test / run (oci, ./frontend/dockerfile, integration)	14m 55s
test / run (oci, ./frontend/dockerfile, dockerfile)	21m 59s
test / run (oci-rootless, ./frontend/dockerfile, integration)	14m 47s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	21m 23s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	21m 13s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	24m 39s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	14m 56s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	21m 46s
image (mainline)	12m 18s
image (labs)	10m 57s
release	0s
scout	0s
	7h 9m 51s

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