GHA: deprioritize Azure Ubuntu mirror

Due to year-long unreliability.

The default Ubuntu mirror works as fast as the Azure one when it's
working at its normal speed. And has HTTPS.

Also:
- replac the retry hack that turn out to not solve the problem.
- add timeouts to each download step to catch slowness early.

Follow-up to a5838847c4 #21181
Follow-up to 5172ba5475 #21107

Closes #21414

.github/workflows/checkdocs.yml

@@ -99,8 +99,10 @@ jobs:
         run: .github/scripts/cleancmd.pl 'docs/*.md'
 
       - name: 'install'
+        timeout-minutes: 2
         run: |
           sudo find /etc/apt/sources.list.d -type f -not -name 'ubuntu.sources' -delete -print
+          sudo sed -i 's/priority:1/priority:9/' /etc/apt/apt-mirrors.txt; cat /etc/apt/apt-mirrors.txt
           sudo apt-get -o Dpkg::Use-Pty=0 update
           sudo apt-get -o Dpkg::Use-Pty=0 install aspell aspell-en
           python3 -m venv ~/venv