pin z3-latest to 4.13.0 for the test case generator

later versions of z3 no longer build on Ubuntu 20.04, so
this commit allows us to keep releasing EverParse binaries
compatible with Ubuntu 20.04.

src/package/package.sh

@@ -260,7 +260,7 @@ make_everparse() {
     # Download and build the latest z3 for test case generation purposes
     if ! $is_windows ; then
         if ! [[ -d z3-latest ]] ; then
-            git clone https://github.com/Z3Prover/z3 z3-latest
+            git clone --branch z3-4.13.0 https://github.com/Z3Prover/z3 z3-latest
         fi
         z3_latest_dir="$PWD/everparse/z3-latest"
         mkdir -p "$z3_latest_dir"