do not use `system` switch by default

[c-cube]

Sorry to start a discussion about that when y'all are busy with opam2. I think the "system" switch is the number 1 source of issues for beginners that try to use opam and come ask for help on #ocaml @ freenode; my advice is almost always to use a non-system switch and it fixes their issue. Therefore I think opam shouldn't use the system switch at all unless explicitly asked so.

[dra27]

There are various ideas in the pipeline for dealing with this once opam 2.0.0 is out of the door. In short, I think it's better to fix the (totally real and confusing) problems with system switches, rather than move towards stopping using them.

[AltGr]

Note that, on opam 2, the automatic switch selection is entirely defined by a customisable formula. So it would be a one-line change to enable/disable selecting system switches by default.

[Drup]

Is there really a point in system switches now ? They cause so many issues and confusion and provide a really bad first impression to new users.

If the only point of system switches is to avoid recompilation, maybe efforts toward making them not suck would be better spend on having compilation cache in a usable state.

[kit-ty-kate]

I opened #6307 to change this default