NRN submodule CI fixes (#883)

* Ubuntu:
  * install pytest(cov) via pip
* macOS
  * SDK_ROOT env
  * use gcc wrapper

Author: alexsavulescu

.github/workflows/test-as-submodule.yml

@@ -28,6 +28,8 @@ jobs:
       fail-fast: false
     env:
       CMAKE_BUILD_PARALLEL_LEVEL: ${{matrix.cores}}
+      SDK_ROOT: $(xcrun --sdk macosx --show-sdk-path)
+
     steps:
 
       - name: Install homebrew packages
@@ -36,16 +38,16 @@ jobs:
           brew install bison coreutils flex ninja openmpi
           python3 -m pip install --upgrade numpy pytest pytest-cov
           echo /usr/local/opt/flex/bin:/usr/local/opt/bison/bin >> $GITHUB_PATH
-          echo "CC=clang" >> $GITHUB_ENV
-          echo "CXX=clang++" >> $GITHUB_ENV
+          echo "CC=gcc" >> $GITHUB_ENV
+          echo "CXX=g++" >> $GITHUB_ENV
 
       - name: Install apt packages
         if: startsWith(matrix.os, 'ubuntu')
         run: |
           sudo apt-get update
           sudo apt-get install bison cython3 flex libfl-dev libopenmpi-dev \
-            ninja-build openmpi-bin python3-dev python3-numpy python3-pytest \
-            python3-pytest-cov
+            ninja-build openmpi-bin python3-dev
+          python3 -m pip install --upgrade numpy pytest pytest-cov
           echo "CC=gcc" >> $GITHUB_ENV
           echo "CXX=g++" >> $GITHUB_ENV