ci, gha: Run "SageMath prover" job on GitHub Actions

.github/workflows/ci.yml

@@ -37,3 +37,19 @@ jobs:
           build\src\RelWithDebInfo\bench_ecmult.exe
           build\src\RelWithDebInfo\bench_internal.exe
           build\src\RelWithDebInfo\bench.exe
+
+  sage:
+    name: "SageMath prover"
+    runs-on: ubuntu-latest
+    container:
+      image: sagemath/sagemath:latest
+      options: --user root
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: CI script
+        run: |
+          cd sage
+          sage prove_group_implementations.sage