make File::try_clone produce non-inheritable handles on Windows
~**NOT READY FOR REVIEW.** This PR is currently mainly to trigger CI so that I can see what happens. (Is there a better way to trigger CI?) I don't know whether this change makes sense yet.~ (Edit: @Mark-Simulacrum clarified that CI doesn't currently run on Windows.)
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File handles shouldn't be inheritable in general.
`std::process::Command` takes care of making them inheritable when child
processes are spawned, and the `CREATE_PROCESS_LOCK` protects against
races in that section on Windows. But `File::try_clone` has been
creating inheritable file descriptors outside of that lock, which could
be leaking into other child processes unintentionally.
See also https://github.com/rust-lang/rust/pull/31069#discussion_r334117665.
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