nixpacks: Include libmysqlclient for Rust's Diesel ORM MySQL Builds

Feature request

The automatic inclusion and linking of libmysqlclient when using diesel orm with mysql.

Motivation

Currently im using diesel with a mysql database, this requires me to have a version of libmysqlclient when building my binaries. Usually i achieve this (on mac) by running brew install mysql-client, then when building the binaries i would set the following env var USTFLAGS="-L/opt/homebrew/opt/mysql-client/lib".

While using railway ive tried installing a similar package however as of current i cant find one that includes libmysqlclient. I did find this but unfortunately i cant install it on railway as its not there.

Contribution

Quite new to rust, and im not sure where to start here tbh as its not just a rust issue i dont think.