buildRustPackage: Restrict `find` to files

`find -executable` finds everything with the executable bit set,
including directories. Thats not harmful in this scenario as `cp` won't
copy those directories, but it does result in a few warning messages.

pkgs/build-support/rust/default.nix

@@ -77,7 +77,7 @@ in stdenv.mkDerivation (args // {
   installPhase = args.installPhase or ''
     runHook preInstall
     mkdir -p $out/bin
-    find target/release -maxdepth 1 -executable -exec cp "{}" $out/bin \;
+    find target/release -maxdepth 1 -executable -type f -exec cp "{}" $out/bin \;
     runHook postInstall
   '';