[ci] update centos7 docker image url

Following a change in the TOS, the dockerhub host is no longer available and the image is now hosted on github instead

.github/workflows/test.yml

@@ -14,10 +14,7 @@ jobs:
     - uses: cvmfs-contrib/github-action-cvmfs@v2
     - name: Start container
       run: |
-        docker run -it --name CI_container -v ${GITHUB_WORKSPACE}:/Package -v /cvmfs:/cvmfs:shared -d clicdp/cc7-lcg /bin/bash
-    - name: Setup container
-      run: |
-        docker exec CI_container /bin/bash -c " ln -s /usr/lib64/liblzma.so.5.2.2 /usr/lib64/liblzma.so;"
+        docker run -it --name CI_container -v ${GITHUB_WORKSPACE}:/Package -v /cvmfs:/cvmfs:shared -d ghcr.io/aidasoft/centos7:latest /bin/bash
     - name: CMake Configure
       run: |
         docker exec CI_container /bin/bash -c 'cd Package;\