github: add --ignore-installed to pip install of requirements

Problem: recently, our github workflow for spelling and
linkcheck got confused between pre-installed ubuntu packages
and python's pip installed packages.

Just ignore the ubuntu packages. Install our own. It doesn't
seem to take much longer.

.github/workflows/main.yml

@@ -38,7 +38,7 @@ jobs:
       run: >-
           sudo apt-get update -y &&
           sudo apt-get install -y python3-pip &&
-          sudo pip3 install --upgrade -r ./requirements.txt
+          sudo pip3 install --ignore-installed --upgrade -r ./requirements.txt
 
     - name: Check Spelling
       uses: crate-ci/typos@7ad296c72fa8265059cc03d1eda562fbdfcd6df2 # v1.9.0