ci: fix backend-sdk-testing repo version

Author: namsnath

.github/workflows/backend-sdk-test.yml

@@ -82,5 +82,5 @@ jobs:
       - uses: supertokens/backend-sdk-testing-action@main
         with:
           # version: ${{ matrix.fdi-version }}
-          version: ci/github-actions/containerized-core/4.0
+          version: ci/github-actions/containerized-core/${{ matrix.fdi-version }}
           path: backend-sdk-testing