Add z3 to Ubuntu 18 package pull request check job

This was previously missing, which would cause any tests which needed Z3
to fail this job.

Author: thomasspriggs

.github/workflows/pull-request-checks.yaml

@@ -434,7 +434,7 @@ jobs:
       - name: Fetch dependencies
         run: | 
           sudo apt-get update
-          sudo apt-get install --no-install-recommends -y g++ flex bison cmake ninja-build maven jq libxml2-utils dpkg-dev ccache
+          sudo apt-get install --no-install-recommends -y g++ flex bison cmake ninja-build maven jq libxml2-utils dpkg-dev ccache z3
           # remove libgcc-s1, which isn't normally available in Ubuntu 18.04
           target=$(dpkg-query -W --showformat='${Version}\n' gcc-8-base | head -n 1)
           # libgcc1 uses an epoch, thus the extra 1: