fix: add native gcc for clamav Rust build scripts in cross-compile

flake.nix

@@ -193,13 +193,11 @@
             "-Dtest_run_result=0"
             "-Dtest_run_result__TRYRUN_OUTPUT="
           ];
-          # Rust proc-macro build scripts need a native linker; cmake overrides
-          # CC/CXX to the cross-compiler, confusing cargo. Set CC to native gcc
-          # so proc-macros can compile and run on the build host.
-          preConfigure = (old.preConfigure or "") + ''
-            export CC=gcc
-            export CXX=g++
-          '';
+          # Give cargo access to a native C compiler for Rust build script
+          # compilations (proc-macros run on the BUILD host, not the target).
+          nativeBuildInputs = (old.nativeBuildInputs or []) ++ [
+            prev.buildPackages.gcc
+          ];
         });
       };