Fine-grained control over `--strict`

[dra27]

The num library's GitHub Actions pipeline performs a test opam install --strict num. It uses --strict in order to test the num.install file generated by its build system is valid (during work on ocaml/num#33, opam install num will merrily install nothing if num.install is corrupt).

The with-dev-setup filter variable added in #5160 / #5214 has started being used in ocaml/opam-repository#24484 which is causing the workflow to fail:

<><> Synchronising pinned packages ><><><><><><><><><><><><><><><><><><><><><><>
[NOTE] Ignoring uncommitted changes in /home/runner/work/num/num (`--working-dir' not active).
[num.1.5~dev] synchronised (git+file:///home/runner/work/num/num#master)

[ERROR] undefined filter variable in dependencies of ambient-context.0.1.0: with-dev-setup
Error: Process completed with exit code 3

This is being worked around in ocaml/num#36 to use the opam 2.2 pre-releases. The problem will of course disappear when opam 2.2 is released, but it would be useful to be able to limit exactly what --strict is applied to.