InstallableFlake: Default attr paths cleanup

This removes some duplicated logic, and fixes "nix bundle" parsing its
installable twice.

src/nix/develop.cc

@@ -498,7 +498,8 @@ struct CmdDevelop : Common, MixEnvironment
                 this,
                 state,
                 installable->nixpkgsFlakeRef(),
-                Strings{"bashInteractive"},
+                "bashInteractive",
+                Strings{},
                 Strings{"legacyPackages." + settings.thisSystem.get() + "."},
                 nixpkgsLockFlags);