Use Apptainer Github Action.

.github/workflows/ubuntu.yml

@@ -11,10 +11,10 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
+        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 
     - name: Set revision
       run: |
@@ -35,6 +35,11 @@ jobs:
         python-version: ${{ matrix.python-version }}
         allow-prereleases: true
 
+    - name: Install Apptainer
+      uses: eWaterCycle/setup-apptainer@v2
+      with:
+        apptainer-version: 1.3.6
+
     - name: Show environment
       run: |
         python --version
@@ -83,13 +88,6 @@ jobs:
         popd
         rm -r runsolver-dir/
 
-    - name: Install Apptainer
-      working-directory: ../deps
-      run: |
-        sudo add-apt-repository -y ppa:apptainer/ppa
-        sudo apt-get update
-        sudo apt-get install -y apptainer
-
     - name: Compile VAL
       working-directory: ../deps
       run: |