buildGraalVmNativeImage: enable utf-8 by default

The tools built with buildGraalVmNativeImage have broken UTF-8 support
when not properly setting the locale, e.g.

$ bb -e '(prn "bépo àê")'
"b??po ????"

This commit sets the locale to en_US.UTF-8 by default, which fixes that.

pkgs/build-support/build-graalvm-native-image/default.nix

@@ -33,6 +33,8 @@ stdenv.mkDerivation (args // {
   nativeImageBuildArgs = nativeImageBuildArgs ++ extraNativeImageBuildArgs ++ [ graalvmXmx ];
 
   buildPhase = args.buildPhase or ''
+    export LC_ALL="en_US.UTF-8"
+
     runHook preBuild
 
     native-image ''${nativeImageBuildArgs[@]}