Fix error format in gio/gunixconnection.c (part 2)

Update a series of error messages to use g_set_error_literal instead of g_set_error. This should prevent format-nonliteral compiler issues when -Werror is configured:

../gio/gunixconnection.c: In function ‘g_unix_connection_receive_fd’:
../gio/gunixconnection.c:183:9: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
  183 |         nscm);
      |         ^~~~
../gio/gunixconnection.c:217:20: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
  217 |                    nfd);
      |                    ^~~
../gio/gunixconnection.c: In function ‘g_unix_connection_receive_credentials’:
../gio/gunixconnection.c:601:24: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
  601 |                        nscm);
      |                        ^~~~

This is similar to a previous change [1] made to gunixconnection.c.

[1]: 44b3d5d8