Let flux auto-upgrade itself

We may want to have flux auto-upgrade itself, so existing clusters can use new helm and flux features without needing a manual upgrade procedure.

This is maybe not completely straightforward, as the standard way to install flux in such a way, using flux bootstrap, is not possible in our read-only setup, see discussion here.

As suggested in that discussion, we could get the same result by installing flux in a "one-shot" way with flux install like we already do, but add kustomizations for managing the flux components through flux itself.