Revisit ci-image

ci-image was created to avoid downloading various components during each run of CI.

we've then also created sylva-toolbox as a component used to standardize the distribution of various binaries required to run sylva core (and even a bit more, for convenience)

Initially, sylva toolbox was not installed in CI as all required binaries were already there, but we've seen that using a different version of some binaries was causing troubles (more specifically, the "old" version of flux that is present in ci-image was preventing flux to bootstrap, see !311 (comment 1351559423))

And more generally, we should be using sylva-toolbox in ci to test what we'll actually deliver to end-users.

As a consequence, we'll be downloading (pretty large) binaries like kubectl or flux twice at each run, which could be avoided. Possible solution could be to use sylva-toolbox as base image for ci-image (but they may be ohters).