Stabilize opam switch when its base package is not available anymore

[kit-ty-kate]

In ocaml/opam-repository#17132 it was realized that the base package of an opam switch can change if it is no longer available.

In the case of this PR, the issue was that the fedora and alpine images for OCaml 4.11 were not updated and still are on 4.11.0+trunk. However since those packages are not valid anymore after the release of 4.11.0, the were moved to 4.11.1+trunk. Opam then realizes that its base package is not available anymore and tries to use the closest base package available:

The following actions will be performed:
  - downgrade ocaml-variants  4.11.0+trunk to 4.11.0+spacetime
  - recompile ocaml-config    1                                [uses ocaml-variants]
  - recompile ocaml           4.11.0                           [uses ocaml-variants]
  - recompile opam-depext     1.1.3                            [uses ocaml]
  - recompile opam-ci-scripts 0.1*                             [uses ocaml]
  - recompile depext          transition                       [uses ocaml]
===== 5 to recompile | 1 to downgrade =====

This PR fixes the issue by pinning the base package once and for all.

This package is a draft for the moment as it requires ocurrent/ocaml-version#18 for an easy way to get the package name and version.

[talex5]

Build is failing. Maybe you need to commit the new submodule version too?

[kit-ty-kate]

All green now (except the slower ppc64 CI ofc)

[talex5]

I've pushed it live for testing.