mumble: Add build option CMAKE_UNITY_BUILD for compilation speed

This provides a significant speed boost to the compilation. Almost 70%
improvement in my experiments.

# plain build
❯ nx hyperfine 'nix build .#mumble --rebuild' --warmup 1
--ignore-failure
Benchmark 1: nix build .#mumble --rebuild
  Time (mean ± σ):     73.287 s ±  2.462 s    [User: 0.679 s, System:
3.338 s]
  Range (min … max):   69.478 s … 77.686 s    10 runs

# unity build
❯ nx hyperfine 'nix build .#mumble --rebuild' --warmup 1
--ignore-failure
Benchmark 1: nix build .#mumble --rebuild
  Time (mean ± σ):     42.713 s ±  1.435 s    [User: 0.089 s, System:
1.216 s]
  Range (min … max):   39.891 s … 44.800 s    10 runs

pkgs/applications/networking/mumble/default.nix

@@ -72,6 +72,7 @@ let
           "-D g15=OFF"
           "-D CMAKE_CXX_STANDARD=17" # protobuf >22 requires C++ 17
           "-D BUILD_NUMBER=${lib.versions.patch source.version}"
+          "-D CMAKE_UNITY_BUILD=ON" # Upstream uses this in their build pipeline to speed up builds
           "-D bundled-gsl=OFF"
           "-D bundled-json=OFF"
         ]