Try re-using container from jenkins

Author: WardBrian

.github/workflows/testing.yml

@@ -19,13 +19,12 @@ jobs:
   run-tests:
     name: run stan tests
     runs-on: ubuntu-24.04
+    container: stanorg/ci:gpu
     steps:
       - name: Check out source code
         uses: actions/checkout@v3
         with:
           submodules:  true
-      - name: Install clang++
-        run: sudo apt-get install clang-${{ env.clangppVersion }}++
       - name: Run tests
         run: |
           echo "CXX=clang++-${{ env.clangppVersion }}" >> make/local
@@ -35,13 +34,12 @@ jobs:
   run-fvar-var-tests:
     name: run stan fvar-var model tests
     runs-on: ubuntu-24.04
+    container: stanorg/ci:gpu
     steps:
       - name: Check out source code
         uses: actions/checkout@v3
         with:
           submodules:  true
-      - name: Install clang++
-        run: sudo apt-get install clang-${{ env.clangppVersion }}++
       - name: Run tests
         run: |
           echo "CXX=clang++-${{ env.clangppVersion }}" >> make/local