ci: update package list before installing asciidoc

On the last two runs we've been getting errors install lxml2-utils (a
dependency of asciidoc) because the version in the packages list isn't
available anymore. A new point build has been pushed.

The base image will be updated eventually but [the docs][] say to run
apt update before installing. That is already done once in this file
too. Yay for longer build times.

Last two runs:

https://github.com/qutebrowser/qutebrowser/runs/5578546699?check_suite_focus=true
https://github.com/qutebrowser/qutebrowser/runs/5581525862?check_suite_focus=true

[the docs]: https://docs.github.com/en/actions/using-github-hosted-runners/customizing-github-hosted-runners#installing-software-on-ubuntu-runners

.github/workflows/ci.yml

@@ -53,7 +53,7 @@ jobs:
       - name: Install dependencies
         run: |
             [[ ${{ matrix.testenv }} == eslint ]] && npm install -g eslint
-            [[ ${{ matrix.testenv }} == docs ]] && sudo apt-get install --no-install-recommends asciidoc
+            [[ ${{ matrix.testenv }} == docs ]] && sudo apt-get update && sudo apt-get install --no-install-recommends asciidoc
             if [[ ${{ matrix.testenv }} == shellcheck ]]; then
                 scversion="stable"
                 bindir="$HOME/.local/bin"