umu-launcher: use steam's FHS profile (#381047)

pkgs/by-name/um/umu-launcher/package.nix

@@ -1,11 +1,20 @@
 {
   buildFHSEnv,
   lib,
+  steam,
   umu-launcher-unwrapped,
   extraPkgs ? pkgs: [ ],
   extraLibraries ? pkgs: [ ],
+  extraProfile ? "", # string to append to shell profile
+  extraEnv ? { }, # Environment variables to include in shell profile
   withMultiArch ? true, # Many Wine games need 32-bit libraries.
 }:
+let
+  # Steam is not a dependency, but we re-use some of its implementation
+  steam' = steam.override {
+    inherit extraEnv extraProfile;
+  };
+in
 buildFHSEnv {
   pname = "umu-launcher";
   inherit (umu-launcher-unwrapped) version meta;
@@ -27,4 +36,10 @@ buildFHSEnv {
     ln -s ${umu-launcher-unwrapped}/lib $out/lib
     ln -s ${umu-launcher-unwrapped}/share $out/share
   '';
+
+  # For umu & proton, we need roughly the same environment as Steam.
+  # For simplicity, we use Steam's `profile` implementation.
+  # See https://github.com/NixOS/nixpkgs/pull/381047
+  # And https://github.com/NixOS/nixpkgs/issues/297662#issuecomment-2647656699
+  inherit (steam'.args) profile;
 }