Support OS version in platform string #5821

Job	Run time
build (windows/amd64)	1m 43s
sandbox-build (linux/amd64)	6m 41s
build (freebsd/amd64)	1m 43s
sandbox-build (linux/arm64)	6m 40s
test-freebsd-amd64	1m 32s
test-windows-amd64 (containerd, ./client)	7m 36s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 38s
test-windows-amd64 (containerd, ./worker/containerd)	3m 57s
test-windows-amd64 (containerd, ./solver)	1m 49s
test-windows-amd64 (containerd, ./frontend)	1m 45s
test-windows-amd64 (containerd, ./frontend/dockerfile)	40m 38s
test-windows-amd64 (./..., 1)	6m 23s
	1h 24m 5s

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