Use binary sources for CI

[gergondet]

This PR:

• adds Ubuntu 20.04 to the CI
• use our apt mirrors to fetch dependencies on Ubuntu builds
• use vcpkg to fetch dependencies on macOS and Windows, with binary caching this cuts down the CI time significantly (from 15-20 minutes to 5-10 minutes)
• fixes a bug where a temporary ends up being used after deletion

Another significant difference (so-far) is that the CI is not building Python bindings on Windows or macOS. I will look into it next week.

I will also consider similar PRs in our other dependencies, I've started with this one because it's complex enough to yield very significant gains and show potential issues (e.g. I had some troubles to get a decent vcpkg port of eigen-qld on Windows)

Note: our vcpkg fork already has some mc_rtc port with some work left to better handle the debug/release co-existing installations