fixup gem install flags

pkgs/development/interpreters/ruby/gem.nix

@@ -45,8 +45,16 @@ in ruby.stdenv.mkDerivation (attrs // {
   installPhase = ''
     runHook preInstall
     GEM_HOME=$out/${ruby.gemPath} \
-      gem install -p http://nodtd.invalid \
-      --build-root / -n "$out/bin" "$src" $gemFlags -- $buildFlags
+      gem install \
+      --local \
+      --force \
+      --http-proxy "http://nodtd.invalid" \
+      --ignore-dependencies \
+      --build-root "/" \
+      --bindir "$out/bin" \
+      --backtrace \
+      $src $gemFlags -- $buildFlags
+
     rm -frv $out/${ruby.gemPath}/cache # don't keep the .gem file here
 
     for prog in $out/bin/*; do