umu-launcher: enable multiArch

[MattSturgeon]

Issue reported by @fufexan: fufexan/nix-gaming#233 (comment)

libGL: Can't open configuration file /etc/drirc: No such file or directory.
libGL: Can't open configuration file /home/mihai/.drirc: No such file or directory.
libGL: using driver amdgpu for 83
libGL: Can't open configuration file /etc/drirc: No such file or directory.
libGL: Can't open configuration file /home/mihai/.drirc: No such file or directory.
libGL: pci id for fd 83: 1002:1638, driver radeonsi
libGL: MESA-LOADER: failed to open /usr/lib/pressure-vessel/overrides/lib/x86_64-linux-gnu/dri/radeonsi_dri.so: /usr/lib/pressure-vessel/overrides/lib/x86_64-linux-gnu/dri/radeonsi_dri.so: wrong ELF class: ELFCLASS64
libGL error: MESA-LOADER: failed to open radeonsi: /usr/lib/pressure-vessel/overrides/lib/x86_64-linux-gnu/dri/radeonsi_dri.so: wrong ELF class: ELFCLASS64 (search paths /usr/lib/pressure-vessel/overrides/lib/x86_64-linux-gnu/dri)
libGL error: failed to load driver: radeonsi
libGL: MESA-LOADER: failed to open /usr/lib/pressure-vessel/overrides/lib/x86_64-linux-gnu/dri/swrast_dri.so: /usr/lib/pressure-vessel/overrides/lib/x86_64-linux-gnu/dri/swrast_dri.so: wrong ELF class: ELFCLASS64
libGL error: MESA-LOADER: failed to open swrast: /usr/lib/pressure-vessel/overrides/lib/x86_64-linux-gnu/dri/swrast_dri.so: wrong ELF class: ELFCLASS64 (search paths /usr/lib/pressure-vessel/overrides/lib/x86_64-linux-gnu/dri)
libGL error: failed to load driver: swrast
X Error of failed request:  GLXBadContext
  Major opcode of failed request:  150 (GLX)
  Minor opcode of failed request:  6 (X_GLXIsDirect)
  Serial number of failed request:  551
  Current serial number in output stream:  550

Resolved by enabling multi-arch in the FHS environment.

This matches the implementation used by Heroic:

nixpkgs/pkgs/games/heroic/fhsenv.nix

Lines 14 to 15 in 6fa3f3d

	# Many Wine and native games need 32-bit libraries.
	multiArch = true;

And the impl previously used by the upstream umu-launcher flake, before it was refactored to override the nixpkgs package:
https://github.com/Open-Wine-Components/umu-launcher/blob/0cac244cc89ee69bf33ad60a3953cfde188ee8a6/packaging/nix/umu-run.nix#L14
(Added by Open-Wine-Components/umu-launcher#252)

cc @LovingMelody

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[fufexan]

Tested osu-stable. It now launches and plays just fine.
LGTM