We want to disable `PLUGIN_AUTH_PAM` when building:
1. `mariadb` on macOS.
2. `mariadb-client` on any platform
Unfortunately, the interaction of these two commits
6c97b0486c7e43b4d0ae
created a situation where we disable it *twice* when building on macOS.
Once in a darwin-specific `prePatch` script, and again in the `patches`
section for client builds.
This removes the redundant `prePatch` script and conditionally applies
the patch to `mariadb` server builds on darwin.
Fixes#70835
b0b0bb7e0b