Initialize plugins after handling initial command line flags

This is technically a breaking change, since attempting to set plugin
files after the first non-flag argument will now throw an error. This
is acceptable given the relative lack of stability in a plugin
interface and the need to tie the knot somewhere once plugins can
actually define new subcommands.

src/nix-env/nix-env.cc

@@ -1420,8 +1420,6 @@ static int main_nix_env(int argc, char * * argv)
 
         myArgs.parseCmdline(argvToStrings(argc, argv));
 
-        initPlugins();
-
         if (!op) throw UsageError("no operation specified");
 
         auto store = openStore();