fixup! [CI] Add a post-install test.

Author: hageboeck

.github/workflows/root-ci.yml

@@ -574,10 +574,11 @@ jobs:
 
       - name: Post-install tests
         run: |
-          cmake -S test/PostInstall/ -B /github/home/ROOT-CI/PostInstall -DCMAKE_PREFIX_PATH=/github/home/ROOT-CI/install -Dtesting=On;
+          cmake -S test/PostInstall/ -B /github/home/ROOT-CI/PostInstall -DCMAKE_PREFIX_PATH=/github/home/ROOT-CI/install;
           cmake --build /github/home/ROOT-CI/PostInstall;
 
-      - run: ctest /github/home/ROOT-CI/PostInstall
+      - working-directory: /github/home/ROOT-CI/PostInstall
+        run: ctest -j $(nproc)
 
   event_file:
     # For any event that is not a PR, the CI will always run. In PRs, the CI