Support OS version in platform string #5853

Job	Run time
sandbox-build (linux/amd64)	7m 3s
build (windows/amd64)	1m 50s
sandbox-build (linux/arm64)	7m 8s
build (freebsd/amd64)	1m 59s
test-freebsd-amd64	2m 36s
test-windows-amd64 (containerd, ./client)	6m 21s
test-windows-amd64 (containerd, ./cmd/buildctl)	4m 10s
test-windows-amd64 (containerd, ./worker/containerd)	3m 40s
test-windows-amd64 (containerd, ./solver)	2m 40s
test-windows-amd64 (containerd, ./frontend)	4m 50s
test-windows-amd64 (containerd, ./frontend/dockerfile)	39m 32s
test-windows-amd64 (./..., 1)	4m 43s
	1h 26m 32s

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