[master] opam init fails when no system compiler is present

[kit-ty-kate]

With opam master (7d926f1, compiled using make cold), on a clean system, without any ocaml compiler available:

[opam@edb63d1833b7 opam]$ ./opam init 
[...]
[WARNING] Shell not updated in non-interactive mode: use --shell-setup
<><> Creating initial switch 'default' (invariant ["ocaml" {>= "4.05.0"}] - initially with ocaml-system) 
<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml" {>= "4.05.0"}]
[ERROR] Could not determine which packages to install for this switch:
  * Missing dependency:
    - (request) → ocaml-system
    unmet availability conditions, e.g. 'sys-ocaml-version = "4.12.0"'
Switch initialisation failed: clean up? ('n' will leave the switch partially installed) [Y/n] y
[NOTE] Opam has been initialised, but the initial switch creation failed.
       Use 'opam switch create <compiler>' to get started.

[rjbou]

Introduced in #4607