A previous commit allowed to select each tool separately on the toolbox.
This commit makes use of that for CI to possibly speed up loading from cachix
a little bit. It will also cause fewer cache misses when nix code is changed.
Resolves#3183
The Lint & Style job needs to run on all PRs, not only when something "test" related
changes. Otherwise not all workflow, nix or other files are style-checked and linted.