CI: explicitly trigger recompilation in axiom check

Due to whatever the CI environment does to cache and restore
intermediate result files, somehow `make validate` triggers some
spurious recompilation. To avoid confusing the checking script,
trigger any possible re-compilation steps before capturing the output
of `make validate`.

.gitlab-ci.yml

@@ -163,6 +163,7 @@ validate:
   dependencies:
     - compile
   script:
+    - make -j ${NJOBS} # CI gotcha — get any spurious recompilation out of the way
     - make validate 2>&1 | tee validation-results.txt
     - scripts/check-validation-output.sh validation-results.txt