CI: Unbreak macOS (#461)

* CI: Unbreak macOS

Try to unbreak broken python on macOS CI..

* Python: Use a fresh Virtualenv

Brew Python is too broken/inconsistent in packages like
`six`, `matplotlib`, `pandas` et al.

.github/workflows/macos.yml

@@ -45,6 +45,12 @@ jobs:
         brew install ninja
         brew install open-mpi
         brew install pkg-config
+
+        python3 -m pip install --upgrade pip
+        python3 -m pip install --upgrade virtualenv
+
+        python3 -m venv py-venv
+        source py-venv/bin/activate
         python3 -m pip install --upgrade pip
         python3 -m pip install --upgrade build packaging setuptools wheel pytest
         python3 -m pip install --upgrade pytest
@@ -64,6 +70,8 @@ jobs:
           ccache-macos-appleclang-
     - name: build ImpactX
       run: |
+        source py-venv/bin/activate
+
         cmake -S . -B build            \
           -DCMAKE_VERBOSE_MAKEFILE=ON  \
           -DImpactX_PYTHON=ON          \
@@ -72,12 +80,15 @@ jobs:
 
     - name: run tests
       run: |
+        source py-venv/bin/activate
+
         ctest --test-dir build --output-on-failure -E pytest.AMReX
 
     - name: run installed python module
       run: |
         cmake --build build --target pip_install
 
+        source py-venv/bin/activate
         python3 examples/fodo/run_fodo.py
 
     - name: run installed app