[actions] retry apt package installs through Ubuntu mirror outages

Mirror outages have outlasted the existing retries,
which covered only `apt-get update` (and not the WSL `additional-packages` install).
Installs now go through `.github/scripts/apt-install.sh`,
which retries each request within apt and retries the whole update+install with backoff for about 10 minutes.

.github/workflows/tests.yml

@@ -54,7 +54,7 @@ jobs:
             azure.archive.ubuntu.com:80
             packages.microsoft.com:443
       - uses: actions/checkout@v6
-      - run: sudo apt-get update; sudo apt-get install ${{ matrix.shell }}
+      - run: sh .github/scripts/apt-install.sh ${{ matrix.shell }}
         if: matrix.shell == 'zsh' || matrix.shell == 'ksh'
         # zsh (https://github.com/actions/runner-images/issues/264) and ksh are not in the ubuntu image
         shell: bash