Support for setting environment variables

[dgdguk]

Description

Some of the libraries used by text-generation-webui have options that can be configured via environment variables. Therefore, it may make sense to add support for environment variables configuration, in the same way that command line arguments are supported by CMD_FLAGS.txt.

Additional Context

This idea came about from an issue with RDNA3 GPUs having high power usage when using llama.cpp backends. The workaround suggested is to set the environment variable GPU_MAX_HW_QUEUES=1, and the functionality seems like it could be useful in general.

As this would be fairly straightforward to implement, I'll put myself forward to implement it. This feature request is more about making sure that the change would be welcome / useful.