Allow us to just set RUST_TARGEt ourselves instead of hacking around in mozilla's
weird custom build system...

--- a/build/moz.configure/rust.configure
+++ b/build/moz.configure/rust.configure
@@ -224,6 +224,8 @@
              rust_supported_targets, arm_target, when=rust_compiler)
     @checking('for rust %s triplet' % host_or_target_str)
     @imports('os')
+    @imports(_from='mozbuild.util', _import='ensure_unicode')
+    @imports(_from='mozbuild.util', _import='system_encoding')
     @imports(_from='mozbuild.configure.util', _import='LineIO')
     @imports(_from='mozbuild.shellutil', _import='quote')
     @imports(_from='tempfile', _import='mkstemp')
@@ -328,7 +330,7 @@
 
             return None
 
-        rustc_target = find_candidate(candidates)
+        rustc_target = os.environ['RUST_TARGET']
 
         if rustc_target is None:
             die("Don't know how to translate {} for rustc".format(
@@ -373,7 +375,7 @@
             os.remove(out_path)
 
         # This target is usable.
-        return rustc_target
+        return ensure_unicode(rustc_target, system_encoding)
 
     return rust_target