cmd/doctor: repair xattr not find

[ylht]

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There is a problem with the current way of checkingxattr command, which is simply running xattr no args and checking if it prints an error. But on macOS 12 beta3, the xattr command with no args would print an error, so the -h command is appended to fix this check.

This type of detection does not affect the previous version and does not require additional unit tests.

[MikeMcQuaid]

Makes sense, thanks!