fix: rename variable to avoid clash with optarg

[PeteAudinate]

https://linux.die.net/man/3/optarg

I realise it's unusual to include both CLI11 and getopt.h, but it can happen in large diverse codebases.

[henryiii]

I think the new name is fine, but a local optarg shouldn't conflict with the extern? Generally only macros (well, one being a macro) collide.

[PeteAudinate]

Thanks for the quick reply.

You're right, it's not really a conflict but MSVC complains:
include\CLI\App.hpp(2847,1): warning C4459: declaration of 'optarg' hides global declaration