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

pkgs/by-name/st/steam/package.nix

@@ -10,11 +10,12 @@
   extraBwrapArgs ? [ ], # extra arguments to pass to bubblewrap (real default is at usage site)
   extraArgs ? "", # arguments to always pass to steam
   extraEnv ? { }, # Environment variables to pass to Steam
+  privateTmp ? true, # if the steam bubblewrap should isolate /tmp
 }:
 let
   steamEnv = { name, runScript, passthru ? {}, meta ? {} }:
   buildFHSEnv {
-    inherit name runScript passthru meta;
+    inherit name runScript passthru meta privateTmp;
 
     multiArch = true;
     includeClosures = true;
@@ -105,8 +106,6 @@ let
       ${extraProfile}
     '';
 
-    privateTmp = true;
-
     inherit extraPreBwrapCmds;
 
     extraBwrapArgs = [