mariadb: fix darwin build

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
  6c97b04
  7e43b4d
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 NixOS#70835

(cherry picked from commit b0b0bb7)

pkgs/servers/sql/mariadb/default.nix

@@ -48,9 +48,6 @@ common = rec { # attributes common to both builds
 
   prePatch = ''
     sed -i 's,[^"]*/var/log,/var/log,g' storage/mroonga/vendor/groonga/CMakeLists.txt
-  '' + optionalString stdenv.hostPlatform.isDarwin ''
-    substituteInPlace cmake/build_configurations/mysql_release.cmake \
-      --replace "SET(PLUGIN_AUTH_PAM YES)" ""
   '';
 
   patches = [
@@ -170,6 +167,8 @@ server = stdenv.mkDerivation (common // {
 
   patches = common.patches ++ [
     ./cmake-without-client.patch
+  ] ++ optionals stdenv.isDarwin [
+    ./cmake-without-plugin-auth-pam.patch
   ];
 
   cmakeFlags = common.cmakeFlags ++ [