ci: add APT retries to prevent transient mirror sync failures

[rkt0209]

Overview

This is a quick stability patch for the newly implemented multi-architecture CI workflow. It resolves a known, transient network issue with the Ubuntu package servers that can cause the emulated QEMU runners (armv7 and riscv64) to randomly fail during the dependency installation phase.

The Issue

Occasionally, the ports.ubuntu.com servers perform a live mirror sync at the exact moment the GitHub Action triggers apt-get update. This causes apt to fetch a package list with a mismatched file size, throwing the following fatal error and halting the CI:
E: Failed to fetch http://ports.ubuntu.com/.../Packages.gz File has unexpected size (618582 != 618585). Mirror sync in progress?

The Fix

By appending -o Acquire::Retries=3 to the apt-get update commands in both the native and emulated blocks, apt will now gracefully back off and retry the connection instead of immediately crashing the build. This makes the CI significantly more resilient to upstream server hiccups.