Fix MSys2 side-conditions for conf-packages, part 3/3

[jmid]

This is the 3rd of 3 PRs fixing MSys2 conf-package side-conditions, following #28939 and #28944.

TLDR: since opam.2.2 MSys uses os = "win32" and os-distribution = "msys2" meaning that the MSys2 side-conditions for conf-packages can be subtly wrong.

This 3rd corrects the following conf-packages:
- conf-postgresql
- conf-rubberband
- conf-sdl2
- conf-sdl2-image
- conf-sdl2-mixer
- conf-sdl2-net
- conf-sdl2-ttf
- conf-sndfile
- conf-sqlite3
- conf-zlib
- conf-zstd

While going over conf-files I noticed conf-rubberband and conf-sndfile weren't pkgconf-testing the installed file (both are MSys2 only), so I added an explicit test to both of them.

[jmid]

I've added the latest two commits to please the linter:

• conf-zstd was missing a conf flag
• the added test in conf-sndfile was referring to, e.g., host-arch-x86_32:installed but missing a corresponding depends field

[mseri]

This should be all right to merge, what do you think?

[jmid]

I agree 👍

CI-wise there are

• another ton of revdeps failures
• conf-file issues to fix on Centos, OpenSuse, and macOS

Again, to avoid too much CI strain, we should address those failures in separate PRs.