[CI] Enable a small post-install CMake build test.

Author: hageboeck

.github/workflows/root-ci.yml

@@ -573,6 +573,18 @@ jobs:
         run: |
           ccache -s || true
 
+      - name: Install
+        run: "cmake --install /github/home/ROOT-CI/build/ --prefix /github/home/ROOT-CI/install"
+
+      - name: Post-install build
+        run: |
+          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;
+
+      - name: Post-install test
+        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
     # can be skipped if the tag [skip-ci] or [skip ci] is written in the title.