refactor: remove manual varlink generated code and instead pull after build time

Author: shanefagan

build.rs

@@ -1,5 +1,7 @@
+// Build script to generate Rust code from Varlink interface definitions.
+// Generated files are placed in OUT_DIR and included in the source via include!.
 fn main() {
-    varlink_generator::cargo_build_tosource("src/contextd.varlink", true);
-    varlink_generator::cargo_build_tosource("src/rgb/observer.varlink", true);
-    varlink_generator::cargo_build_tosource("src/rgb/control.varlink", true);
+    varlink_generator::cargo_build("src/contextd.varlink");
+    varlink_generator::cargo_build("src/rgb/observer.varlink");
+    varlink_generator::cargo_build("src/rgb/control.varlink");
 }