steam: Allow overriding privateTmp

Some programs use /tmp to communicate, and steam having a private tmp
causes issues with that. Allow disabling privateTmp in an override.

Things that rely on /tmp:
  - PROTON_DUMP_DEBUG_COMMANDS dumps files to /tmp, where they can't
    be retrieved because steam-run gets a different /tmp
  - I'm not sure why this is, but in my testing, running tasklist
    through proton + cmd.exe would only show running steam games
    when privateTmp was false. I wanted this to get the windows
    pid of a steam game for attaching to it with Textractor.
  - Communication between teamspeak/arma 3 as reported in
    https://github.com/NixOS/nixpkgs/issues/381923

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