[WIP] Use env context to set directories for post-install tests.

Author: hageboeck

.github/workflows/root-ci.yml

@@ -420,6 +420,11 @@ jobs:
         CONTAINER_IMAGE: "registry.cern.ch/root-ci/${{ matrix.image }}:buildready" #KEEP IN SYNC WITH ABOVE
         CONTAINER_OPTIONS: "--security-opt label=disable --rm ${{ matrix.property == 'gpu' && '--device nvidia.com/gpu=all' || '' }}" #KEEP IN SYNC WITH ABOVE
 
+    env:
+      BUILD_DIR: /github/home/ROOT-CI/build
+      INSTALL_DIR: /github/home/ROOT-CI/install
+      POST_INSTALL_DIR: /github/home/ROOT-CI/PostInstall
+
     steps:
       - name: Configure large ccache
         if: ${{ matrix.is_special }}
@@ -564,19 +569,19 @@ jobs:
           ccache -s || true
 
       - name: Install
-        run: "cmake --install /github/home/ROOT-CI/build/ --prefix /github/home/ROOT-CI/install"
+        run: "cmake --install ${{ env.BUILD_DIR }} --prefix ${{ env.INSTALL_DIR }}"
 
       - 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;
+          cmake -S test/PostInstall/ -B ${{ env.POST_INSTALL_DIR }} -DCMAKE_PREFIX_PATH=${{ env.INSTALL_DIR }};
+          cmake --build ${{ env.POST_INSTALL_DIR }};
 
       - name: Post-install test
-        working-directory: /github/home/ROOT-CI/PostInstall
+        working-directory: ${{ env.POST_INSTALL_DIR }}
         run: ctest -j $(nproc)
 
       - name: Check installed headers
-        run: bash test/PostInstall/check-headers.sh /github/home/ROOT-CI/install/include
+        run: bash test/PostInstall/check-headers.sh ${{ env.INSTALL_DIR }}/include
 
   event_file:
     # For any event that is not a PR, the CI will always run. In PRs, the CI