Motivation for this change

Set the default socket location to match the one used by the MySQL module, so PHP programs using pdo_mysql don't have to specify this themselves. This used to work before #83896 was merged, since the module automatically picked up the value set by --with-mysql-sock= at compile time - this parameter is not available for pdo_mysql, though, so we'll have to set the variable directly instead.

This fixes the issues experienced in #84952, where the default config only specifies localhost for the MySQL host address.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.