Fix passing meta attribute to buildenv.nix

Since the meta attributes were not sorted, attribute lookup could
fail, leading to package priorities and active flags not working
correctly.

Broken since 0f24400d90.

src/nix-env/user-env.cc

@@ -91,6 +91,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
             if (!v) continue;
             vMeta.attrs->push_back(Attr(state.symbols.create(*j), v));
         }
+        vMeta.attrs->sort();
         v.attrs->sort();
 
         if (drvPath != "") references.insert(drvPath);