Delete the EXTERNALLY-MANAGED marker file from dolfinx container to a…

…void having to pass --break-system-packages to every pip install call

.github/workflows/website.yml

@@ -31,6 +31,7 @@ jobs:
               export DEBIAN_FRONTEND="noninteractive"
               apt update -y -q
               apt install -y -qq xvfb
+              rm /usr/lib/python3.*/EXTERNALLY-MANAGED
             notebook_pattern: |
               "**/tutorial_*_dolfinx.ipynb"
           - backend: firedrake