Remove pip update command from "Check Website" workflow

I'm actually not sure that this is an improvement, but it's this way in the template, so it's best to just sync it for
now and hopefully take a closer look at some point in the future.

Author: per1234

.github/workflows/check-mkdocs-task.yml

@@ -44,9 +44,7 @@ jobs:
           python-version: "3.9"
 
       - name: Install Poetry
-        run: |
-          python -m pip install --upgrade pip
-          python -m pip install poetry
+        run: pip install poetry
 
       - name: Install Task
         uses: arduino/setup-task@v1