This allows `termio.Exec` to track processes spawned via
`FlatpakHostCommand`, finally allowing Ghostty to function as a
Flatpak.
Alongside this is a few bug fixes:
* Don't add ghostty to PATH when running in flatpak mode since it's
unreachable.
* Correctly handle exit status returned by Flatpak. Previously this was
not processed and contains extra status bits.
* Use correct type for PID returned by Flatpak.
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