Try with bash variable instead of GitHub context

Signed-off-by: Iyán Méndez Veiga <[email protected]>

.github/workflows/basic.yml

@@ -116,7 +116,7 @@ jobs:
         working-directory: ${{ env.RANDOM_BUILD_DIR }}
       - name: Link with C++ program
         run: |
-          g++ ${{ env.GITHUB_WORKSPACE }}/cpp/sig_linking_test.cpp -g \
+          g++ "$GITHUB_WORKSPACE"/cpp/sig_linking_test.cpp -g \
           -I./include -L./lib -loqs -lcrypto -std=c++11 -o example_sig && \
           ./example_sig
         working-directory: ${{ env.RANDOM_BUILD_DIR }}