Open the store before constructing EvalState.

EvalState requires the `store` global to be initialized before it is constructed
in some cases, e.g. when it needs to download a tarball for something in
NIX_PATH. Hence, this fixes #13.

nix-repl.cc

@@ -78,8 +78,6 @@ NixRepl::NixRepl(const Strings & searchPath)
     , staticEnv(false, &state.staticBaseEnv)
 {
     curDir = absPath(".");
-
-    store = openStore();
 }
 
 
@@ -622,6 +620,7 @@ int main(int argc, char * * argv)
             return true;
         });
 
+        store = openStore();
         NixRepl repl(searchPath);
         repl.mainLoop(files);
     });