un-do '--use-internal-solver'

It leads to ocamlfind being upgraded and downgraded all the time, which leads to
Coq being recompiled.

prepare-opam.sh

@@ -70,7 +70,7 @@ fi
 
 # Install build-dependencies.
 echo_color "$BOLDGREEN" "[prepare-opam] Installing build-dependencies"
-make build-dep OPAMFLAGS="-y --use-internal-solver"
+make build-dep OPAMFLAGS="-y"
 echo
 
 # done