[llvm] workflows: Run `apt update` before `apt install` (PR #67136)

[David Spickett via llvm-commits]

DavidSpickett wrote:

This workflow got refactored in https://github.com/llvm/llvm-project/pull/89521, and as far as I can tell does not apt install anything. Or if it does, it's done in another script.

https://github.com/llvm/llvm-project/pull/67136