Enlarge find_other_exec's meager fgets buffer

The buffer was 100 bytes long, which is barely sufficient when the
version string gets longer (such as by configure --with-extra-version).
Set it to MAXPGPATH.

Author: Nikhil Sontakke
Discussion: https://postgr.es/m/CAMGcDxfLfpYU_Jru++L6ARPCOyxr0W+2O3Q54TDi5XdYeU36ow@mail.gmail.com