MacOS CI: install pkgconf instead of pkg-config

[SkySkimmer]

Hopefully fix CI

(pkgconf is a replacement of pkg-config)

cf ocaml/opam-repository#26891

[SkySkimmer]

not sure if this was doing anything

[SkySkimmer]

this seems to be working even though it's a strange back-and-forth

note that the image comes with pkg-config preinstalled AFAICT

History:
With the previous install pkg-config we get error https://github.com/coq/coq/actions/runs/11856721924/job/33043567199?pr=19835

The following system packages will first need to be installed:
    pkgconf

if we want to install pkgconf we must unlink pkg-config
then with unlink pkg-config; install pkgconf we get https://github.com/coq/coq/actions/runs/11857007213/job/33044451483

ERROR while compiling conf-pkg-config.3
# [ERROR] Command not found: pkg-config

If we install pkgconf but unlink it in favor of pkg-config as in the current code it seems to work

[mseri]

One solution would be to opam pin conf-pkg-config.2 when on a macos runner, then you don’t need any other change.

At some point the image will be updated with an updated homebrew cache I guess, then the pin will fail and you can remove it

[mseri]

Sorry for the breakage, by the way. Even if it would have happened also by releasing a version 4 of the conf package, it may have been clearer that way

[proux01]

Let's merge as it seems to work, this can be potentially refine dlater if needed.

[proux01]

@coqbot merge now