Don't use read-only mode for nix build --dry-run

In dry run mode, new derivations can't be create, so running the command on anything that has not been evaluated before results in an error message of the form `don't know how to build these paths (may be caused by read-only store access)`.

For comparison, the classical `nix-build --dry-run` doesn't use read-only mode.

Closes NixOS#1795

src/nix/installables.cc

@@ -257,9 +257,6 @@ std::shared_ptr<Installable> parseInstallable(
 Buildables build(ref<Store> store, RealiseMode mode,
     std::vector<std::shared_ptr<Installable>> installables)
 {
-    if (mode != Build)
-        settings.readOnlyMode = true;
-
     Buildables buildables;
 
     std::vector<StorePathWithOutputs> pathsToBuild;