ci: Install gi-docgen using apt

On Ubuntu jobs, instead of installing pip then installing gi-docgen
with pip, simply install it using apt.

.github/workflows/build.yml

@@ -24,8 +24,7 @@ jobs:
       - name: Install dependencies
         run: |
           apt-get update
-          apt-get install -y libglib2.0 gettext dbus meson libgirepository1.0-dev libgtk-3-dev libgtk-4-dev valac python3-pip python3-dbusmock
-          pip3 install gi-docgen
+          apt-get install -y libglib2.0 gettext dbus meson gi-docgen libgirepository1.0-dev libgtk-3-dev libgtk-4-dev valac python3-dbusmock
           echo "$HOME/.local/bin" >> $GITHUB_PATH
       - name: Check out libportal
         uses: actions/checkout@v3