nix-env: Refuse to operate on a new-style profile

This prevents users from accidentally nuking their profile via
nix-env.

src/nix-env/user-env.cc

@@ -15,6 +15,8 @@ namespace nix {
 DrvInfos queryInstalled(EvalState & state, const Path & userEnv)
 {
     DrvInfos elems;
+    if (pathExists(userEnv + "/manifest.json"))
+        throw Error("profile '%s' is incompatible with 'nix-env'; please use 'nix profile' instead", userEnv);
     Path manifestFile = userEnv + "/manifest.nix";
     if (pathExists(manifestFile)) {
         Value v;