nix/develop: Fix misleading ignored error when run with --arg/--argstr

This would print erroneous and misleading diagnostics like:

> error (ignored): error: '--arg' and '--argstr' are incompatible with flakes

When run with --expr/--file. Since this installable is used to get the
bash package it doesn't make sense to check this.

(cherry picked from commit b6f98b52a4)

src/nix/develop.cc

@@ -647,7 +647,7 @@ struct CmdDevelop : Common, MixEnvironment
                 nixpkgs = i->nixpkgsFlakeRef();
 
             auto bashInstallable = make_ref<InstallableFlake>(
-                this,
+                nullptr, //< Don't barf when the command is run with --arg/--argstr
                 state,
                 std::move(nixpkgs),
                 "bashInteractive",