phantomjs2: set QT_QPA_PLATFORM to allow use in daemons

resolves issues with Grafana email attachments

Suggested in https://github.com/ariya/phantomjs/issues/15217#issuecomment-354713760

pkgs/development/tools/phantomjs2/default.nix

···
     $out/bin/phantomjs
   '' + ''
     wrapProgram $out/bin/phantomjs \
+    --set QT_QPA_PLATFORM offscreen \
     --prefix PATH : ${stdenv.lib.makeBinPath [ qtbase ]}
   '';