GHA: sync apt-get code between workflows/jobs

- delete 3rd-party apt sources, where missing.
- do `apt-get update`, where missing.

Closes #22067

.github/workflows/non-native.yml

@@ -418,7 +418,9 @@ jobs:
       - name: 'install packages'
         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 libfl2
 
       - name: 'cache compiler (djgpp)'