Update docs.yml

use conda to install graphviz

Author: tomvanmele

.github/workflows/docs.yml

@@ -17,15 +17,22 @@ jobs:
     env:
       GH_PAT: ${{secrets.GH_PAT}}
     steps:
+
       - uses: actions/checkout@v2
-      - name: Set up Python 3.8
-        uses: actions/setup-python@v2
+
+      - uses: conda-incubator/setup-miniconda@v2.0.0
         with:
+          miniconda-version: "latest"
+          activate-environment: compas-dev
+          channels: conda-forge
           python-version: 3.8
+
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
           python -m pip install cython --install-option="--no-cython-compile"
+          conda install graphviz
+
       - name: Install
         run: |
           python -m pip install --no-cache-dir -r requirements-dev.txt
@@ -92,4 +99,4 @@ jobs:
           build_dir: ./
           keep_history: false  
         env:
-          GH_PAT: ${{ secrets.GH_PAT }}
+          GH_PAT: ${{ secrets.GH_PAT }}