ci: check if can skip installing texlive

.github/workflows/sphinx-build-pdf.yaml

@@ -5,6 +5,7 @@ on: [push, pull_request]
 jobs:
   build:
     runs-on: ubuntu-latest
+    container: readthedocs/build:6.0
     strategy:
       fail-fast: false
     steps:
@@ -18,5 +19,4 @@ jobs:
         pip install -r requirements.txt
     - name: Sphinx build - pdf
       run: |
-        sudo apt-get update && sudo apt-get -y --no-install-recommends install latexmk texlive-full
         make latexpdf