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-copy-closure/nix-copy-closure.cc

@@ -43,8 +43,6 @@ static int main_nix_copy_closure(int argc, char ** argv)
             return true;
         });
 
-        initPlugins();
-
         if (sshHost.empty())
             throw UsageError("no host name specified");