Force Kali Linux to use a single APT mirror for install and update

[alexleigh]

This avoids an issue where apt-get update and apt-get install commands hit different mirrors with different versions of packages, causing install to fail.

This should fix the DNS workflow for https://github.com/acmesh-official/acme.sh/pull/4542

[alexleigh]

Kali Linux repos have since updated and they are now consistent with each other, so this problem no longer appears. However, this change can still be useful in case it happens again in the future.

The problem arises when the Kali mirror chosen by Github actions for apt-get update has older versions of packages that have been deleted from other mirrors. apt-get install chooses one of these other mirrors to download the packages needed by the test workflow, which then fails because those packages are gone. Forcing a single mirror as the apt source avoids this issue.