ci: couple of potential improvements

While porting zbus's ci to fdo template I experimented with a couple of changes:

• Installing Rust stable and nightly on the same image, using rustup override set $x in each job to pick the version we want. This would help simplifying our ci setup as we'd have only one image instead of 3 (4 including the base one).

• Calling cargo fetch when generating the image so build deps are part of the image, saving each job to re-download them. Only worth if images are refreshed frequently enough that those deps stay relevant. Shouldn't be a problem if we regenerate the image each week for nightly I think.

We can wait a bit to see how it works for zbus and then consider doing the same for our ci here.