Support OS version in platform string #5822

Job	Run time
build (windows/amd64)	1m 44s
build (freebsd/amd64)	1m 52s
sandbox-build (linux/amd64)	6m 46s
sandbox-build (linux/arm64)	6m 38s
test-freebsd-amd64	3m 5s
test-windows-amd64 (containerd, ./client)	8m 21s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 32s
test-windows-amd64 (containerd, ./worker/containerd)	3m 21s
test-windows-amd64 (containerd, ./solver)	2m 57s
test-windows-amd64 (containerd, ./frontend)	2m 46s
test-windows-amd64 (containerd, ./frontend/dockerfile)	18m 51s
test-windows-amd64 (./..., 1)	4m 13s
	1h 4m 6s

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