unnethack: disable build parallelism
Without the change the build fails occasionally as:
gcc -DAUTOCONF -Wall -I../include -I./../include -g -O2 -c ./../sys/unix/unixmain.c
./../sys/unix/unixmain.c:9:10: fatal error: date.h: No such file or directory
9 | #include "date.h"
| ^~~~~~~~