fix schur complement GHA workflow (#492)

Author: bknueven

Diff for: .github/workflows/test_pr_and_main.yml

@@ -169,17 +169,19 @@ jobs:
       matrix:
         python-version: [3.9]
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v3
     - name: setup conda
-      uses: s-weigand/setup-conda@v1
+      uses: conda-incubator/setup-miniconda@v2
       with:
-        update-conda: true
         python-version: ${{ matrix.python-version }}
-        conda-channels: anaconda, conda-forge
+        channels: anaconda, conda-forge
+        activate-environment: test_env
+        auto-activate-base: false
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
         pip install nose pybind11
+        conda install conda-forge::libstdcxx-ng
         conda install openmpi pymumps --no-update-deps
         pip install mpi4py pandas
         pip install git+https://github.com/pyutilib/pyutilib.git