plugin: switch from ld.gold to mold (cxx-qt docs recommend moving away from gold)

nix/plugin.nix

@@ -1,6 +1,7 @@
 {
   lib,
   craneLib,
+  mold,
   pkg-config,
   qt6,
   writeShellScript,
@@ -67,6 +68,7 @@ let
 
     nativeBuildInputs = [
       pkg-config
+      mold
       qt6.qtbase
       qt6.qtdeclarative
     ];
@@ -79,8 +81,12 @@ let
 
     # qt6.qtbase's setup hook overrides QMAKE after derivation attrs are set,
     # so re-assert it in preBuild which runs after all setup hooks.
+    # Force mold as the linker (cxx-qt book deprecates ld.gold). Setting via
+    # CARGO_TARGET_*_LINKER + RUSTFLAGS is the only path that survives nixpkgs'
+    # cc-wrapper - `-fuse-ld=` from build.rs gets eaten by the wrapper.
     preBuild = ''
       export QMAKE=${qmakeWrapper}
+      export RUSTFLAGS="-C link-arg=-fuse-ld=mold ''${RUSTFLAGS:-}"
     '';
   };