mop: fix build on bash-5

After 33518fcb453382 "stdenv/setup.sh: fix read -N 0 for bash 5"
build started failing as:

consumeEntire(): ERROR: Input null bytes, won't process

Let's avoid running substituteInPlace on arbitrary binary files.

pkgs/applications/misc/mop/default.nix

···
   goDeps = ./deps.nix;
 
   preConfigure = ''
-    for i in $(find . -type f);do
+    for i in *.go **/*.go; do
         substituteInPlace $i --replace michaeldv/termbox-go nsf/termbox-go
     done
     substituteInPlace Makefile --replace mop/cmd mop/mop