Prevent 'opam switch create <version>' from looking for packages mentioning the sys-ocaml-version variable

[kit-ty-kate]

Proposal 2 for making opam switch create <version> avoid ocaml-system coherently with the new behaviour of opam init introduced in #6307
Mentioned in #6407 (comment)
Proposal 1: #6465

Pros:

• Does not affect custom repositories like Proposal 1
• Simpler implementation compared to Proposal 1

Cons:

• There isn't really any way to prevent this behaviour for people who want the old one, except if were to add an ugly option just for this