update the github action

.github/workflows/BuildTest.yml

@@ -21,6 +21,8 @@ jobs:
         uses: actions/checkout@v2
       - run: echo "💡 The ${{ github.repository }} repository has been cloned to the runner."
       - run: echo "🖥️ The workflow is now ready to test your code on the runner."
+      - name: Set up GSL library
+        run: sudo apt-get install libgsl-dev
       - name: Build iSS
         run: |
           mkdir build