[CI] Add --pre option to sample installation step

This is required to find the version of Cantera in the dist folder. By
default, pip only finds stable versions, and in this case, was going out
to PyPI to find Cantera rather than use the local artifact. However, we
can't set --no-index because we also need to install other dependencies
from PyPI.

.github/workflows/main.yml

@@ -366,6 +366,10 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
           architecture: x64
+      - name: Install Apt dependencies
+        run: |
+          sudo apt update
+          sudo apt install graphviz
       - name: Download the wheel artifact
         uses: actions/download-artifact@v3
         with:
@@ -374,7 +378,8 @@ jobs:
       - name: Upgrade pip
         run: python3 -m pip install -U pip setuptools wheel
       - name: Install Python dependencies
-        run: python3 -m pip install --find-links dist cantera h5py pandas matplotlib scipy pint
+        run: python3 -m pip install --pre --find-links dist cantera h5py pandas \
+             matplotlib scipy pint
       - name: Run the examples
         # See https://unix.stackexchange.com/a/392973 for an explanation of the -exec part
         run: |
@@ -382,7 +387,6 @@ jobs:
           find samples/python -type f -iname "*.py" \
           -exec sh -c 'for n; do echo "$n" | tee -a results.txt && python3 "$n" >> results.txt || exit 1; done' sh {} +
         env:
-          PYTHONPATH: build/python
           # The ignore setting here is due to a new warning introduced in Matplotlib==3.6.0
           PYTHONWARNINGS: "error,ignore:warn_name_set_on_empty_Forward::pyparsing"
           MPLBACKEND: Agg