fix: set CC=gcc for clamav Rust proc-macro builds in cross-compile

flake.nix

@@ -193,6 +193,13 @@
             "-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++
+          '';
         });
       };