nix run: Fix accidental removal of /nix/store existence check

Parenthetical to #1686, we don't need to create a new root if we can
just bind-mount on top of the existing /nix/store.

src/nix/run.cc

@@ -184,7 +184,7 @@ void chrootHelper(int argc, char * * argv)
        but that doesn't work in a user namespace yet (Ubuntu has a
        patch for this:
        https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1478578). */
-    if (true /* !pathExists(storeDir) */) {
+    if (!pathExists(storeDir)) {
         // FIXME: Use overlayfs?
 
         Path tmpDir = createTempDir();