tests: skip TestContextChangeDirToFile on Windows #5015

Job	Run time
build (windows/amd64)	1m 37s
build (freebsd/amd64)	1m 40s
test-freebsd-amd64	3m 31s
test-windows-amd64 (containerd, ./client)	3m 40s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 53s
test-windows-amd64 (containerd, ./worker/containerd)	2m 37s
test-windows-amd64 (containerd, ./solver)	3m 12s
test-windows-amd64 (containerd, ./frontend)	2m 30s
test-windows-amd64 (containerd, ./frontend/dockerfile)	29m 1s
test-windows-amd64 (./..., 1)	9m 43s
	1h 0m 24s

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