As discussed in the Staging room on Matrix recently.
This could probably be a bit bigger than it is: perhaps building
packages like `hello`, `hello-cpp`, and `coreutils`, making sure we
exercise CMake and Meson, and possibly building Rust too. But I’ve
left it at the bare minimum for now since this seems like an obvious
Schelling point for what to build on `staging` and keeps any worries
about Hydra load or cache size growth to a minimum.
Closes: #43618
