CI: update gcc to 12

CI: update gcc to 12

gcc 10 has false positives around null strings:

In file included from /usr/include/stdio.h:894,
                 from lib/strutils.c:8:
In function ‘printf’,
    inlined from ‘test_strutils_cstrcasecmp’ at lib/strutils.c:1324:2,
    inlined from ‘main’ at lib/strutils.c:1348:10:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:112:10: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
  112 |   return __printf_chk (__USE_FORTIFY_LEVEL - 1, __fmt, __va_arg_pack ());
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors