Cache APT packages

[PeterBowman]

Some time ago, I was fiddling with the following GH actions:

• https://github.com/tecolicom/actions-use-apt-tools
• https://github.com/awalsh128/cache-apt-pkgs-action

The point of these is to avoid repeatedly fetching and installing the same APT packages on every CI job run. However, I'm concerned about how those actions handle package updates. Frequent changes in the APT repository might render this idea pointless.