Because the macOS CLI runs in the sandbox, including the filesystem,
so users would be confused that -cpu-profile=prof.cpu succeeds but doesn't
write to their current directory, but rather in some random Library/Containers
directory somewhere on the machine (which varies depending on the Mac build
type: App Store vs System Extension)
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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