os-posix: Clean up global variable shadowing

os-posix: Clean up global variable shadowing

Fix:

  os-posix.c:103:31: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
  bool os_set_runas(const char *optarg)
                                ^
  os-posix.c:176:32: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
  void os_set_chroot(const char *optarg)
                                 ^
  /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/getopt.h:77:14: note: previous declaration is here
  extern char *optarg;                    /* getopt(3) external variables */
               ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231004120019.93101-5-philmd@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>