ci: use main branch for docs upstream validation workflow #5334

Job	Run time
prepare	1s
test / prepare	2m 2s
test / run (containerd, ./frontend/dockerfile, integration)	5m 37s
test / run (containerd, ./frontend/dockerfile, dockerfile)	9m 15s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	5m 6s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	9m 6s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 45s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	9m 20s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	5m 10s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	9m 27s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 38s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	9m 27s
test / run (oci, ./frontend/dockerfile, integration)	4m 47s
test / run (oci, ./frontend/dockerfile, dockerfile)	8m 13s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 54s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	8m 25s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 42s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	8m 41s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 22s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	8m 15s
image (mainline)	7m 17s
image (labs)	7m 14s
scout	0s
release	0s
	2h 23m 44s

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