Update build_docs.yml

added libgl req

Author: C-Achard

.github/workflows/build_docs.yml

@@ -11,6 +11,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+    - run: apt-get update && apt-get install libgl1
     - uses: actions/checkout@v1
     # Standard drop-in approach that should work for most people.
     - uses: ammaraskar/sphinx-action@master