Without the change `unnethack` startup crashes as:

    (gdb) bt
    #0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
    #1  0x00007f734250c0e3 in __pthread_kill_internal (signo=6, threadid=<optimized out>) at pthread_kill.c:78
    #2  0x00007f73424bce06 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
    #3  0x00007f73424a58f5 in __GI_abort () at abort.c:79
    #4  0x00007f73424a67a1 in __libc_message (fmt=fmt@entry=0x7f734261e2f8 "*** %s ***: terminated\n") at ../sysdeps/posix/libc_fatal.c:150
    #5  0x00007f734259b1d9 in __GI___fortify_fail (msg=msg@entry=0x7f734261e2df "buffer overflow detected") at fortify_fail.c:24
    #6  0x00007f734259ab94 in __GI___chk_fail () at chk_fail.c:28
    #7  0x00000000005b2ac5 in strcpy (__src=0x7ffe68838b00 "Shall I pick a character's race, role, gender and alignment for you? [YNTQ] (y)",
        __dest=0x7ffe68838990 "\001") at /nix/store/B0S2LKF593R3585038WS4JD3LYLF2WDX-glibc-2.38-44-dev/include/bits/string_fortified.h:79
    #8  curses_break_str (str=str@entry=0x7ffe68838b00 "Shall I pick a character's race, role, gender and alignment for you? [YNTQ] (y)", width=width@entry=163,
        line_num=line_num@entry=1) at ../win/curses/cursmisc.c:275
    NixOS#9  0x00000000005b3f51 in curses_character_input_dialog (prompt=prompt@entry=0x7ffe68838cf0 "Shall I pick a character's race, role, gender and alignment for you?",
        choices=choices@entry=0x7ffe68838d70 "YNTQ", def=def@entry=121) at ../win/curses/cursdial.c:211
    NixOS#10 0x00000000005b9ca0 in curses_choose_character () at ../win/curses/cursinit.c:556
    NixOS#11 0x0000000000404eb1 in main (argc=<optimized out>, argv=<optimized out>) at ./../sys/unix/unixmain.c:309

which corresponds to `gcc` warning:

    ../win/curses/cursmisc.c: In function 'curses_break_str':
    ../win/curses/cursmisc.c:275:5: warning: '__builtin___strcpy_chk' writing one too many bytes into a region of a size that depends on 'strlen' [-Wstringop-overflow=]
      275 |     strcpy(substr, str);
          |     ^

I did not find a single small upstream change that fixes it. Let's
disable `fortify3` until next release.

Closes: NixOS#292113