fix gihub workflow yaml to use correct syntacx for accessing env vars

Author: dbarker

.github/workflows/ci.yml

@@ -555,12 +555,12 @@ jobs:
         sudo ./ci/setup_grpc.sh -r $GRPC_VERSION -s $CXX_STANDARD -p protobuf
     - name: build and run tests
       env:
-        OTEL_CPP_TEST_INSTALL_DIR: "$HOME/test-install"
+        OTEL_CPP_TEST_INSTALL_DIR: "${{ env.HOME }}/test-install"
       run: |
         ./ci/do_ci.sh cmake.install.test
     - name: verify packages
       env:
-        PKG_CONFIG_PATH: "$HOME/test-install/lib/pkgconfig:${PKG_CONFIG_PATH:-}"
+        PKG_CONFIG_PATH: "${{ env.HOME }}/test-install/lib/pkgconfig:${{ env.PKG_CONFIG_PATH }}"
       run: |
         ./ci/verify_packages.sh