[compiler] [build-fs] Fix target not set error

Regression from #1655 .

Not sure if `Config.target` should be better handled as a parameter?

Author: ejgallego

compiler/bin-js_of_ocaml/build_fs.ml

@@ -61,6 +61,7 @@ function jsoo_create_file_extern(name,content){
 }
 |}
   in
+  Config.set_target `JavaScript;
   let fragments = Linker.Fragment.parse_string code in
   Linker.load_fragments ~target_env:Isomorphic ~filename:"<dummy>" fragments;
   Linker.check_deps ();