mmake gets smarter about locating make

- Check `$COSMOCC`, defaulting to `/opt/cosmocc`.

- Try to get the full path of the repo `make.com`. I'm not aware of
  a way of getting the path that defines a zsh function, so the best
  fallback available is `$PWD`.

tool/zsh/mmake

@@ -38,7 +38,7 @@ done
   whence nproc >/dev/null || autoload -Uz nproc
   j=-j$(nproc)
 }
-local make=${MAKE:-$(echo /opt/cosmocc/bin/make(X) 2>/dev/null)}
-[[ -z $make ]] && make=build/bootstrap/make.com
+local make=${MAKE:-${COSMOCC:-/opt/cosmocc}/bin/make}
+[[ -x $make ]] || make=${COSMO:-$PWD}/build/bootstrap/make.com
 ( set -x
   exec $make $j $flags MODE=$mode $targs )