nix: build nova-plugin via crane to cache deps separately from crate src

plugin/build.rs

@@ -8,7 +8,15 @@ fn main() {
     // plugin loader finds nothing exported. --export-dynamic re-exposes them.
     println!("cargo:rustc-link-arg=-Wl,--export-dynamic");
 
+    // Crane's deps-only build stubs out lib.rs and removes our bridge sources to
+    // build a dummy crate that only compiles dependencies. Skip cxx-qt codegen in
+    // that case - we just want cxx-qt-lib (the heavy dep) cached, not our own glue.
+    let bridge_files = ["src/system_stats.rs", "src/cpu_service.rs"];
+    if !bridge_files.iter().all(|p| std::path::Path::new(p).exists()) {
+        return;
+    }
+
     CxxQtBuilder::new_qml_module(QmlModule::new("NovaStats").plugin_type(PluginType::Dynamic))
-        .files(["src/system_stats.rs", "src/cpu_service.rs"])
+        .files(bridge_files)
         .build();
 }