cmd/buildkitd: move reexec.Init to non-Linux files #5860

Job	Run time
build (windows/amd64)	1m 47s
build (freebsd/amd64)	1m 44s
sandbox-build (linux/amd64)	6m 55s
sandbox-build (linux/arm64)	6m 48s
test-windows-amd64 (containerd, ./client)	7m 24s
test-freebsd-amd64	2m 53s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 0s
test-windows-amd64 (containerd, ./worker/containerd)	1m 56s
test-windows-amd64 (containerd, ./solver)	3m 23s
test-windows-amd64 (containerd, ./frontend)	3m 0s
test-windows-amd64 (containerd, ./frontend/dockerfile)	14m 50s
test-windows-amd64 (./..., 1)	5m 17s
	58m 57s

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