Allow environment variables to be evaluated by the preprocessor

[kumavale]

Fixes #2040

Evaluate some environment variables ($ENV_VARS) in the script's path. Strings surrounded by % and % are treated as environment variables for Windows, and strings starting with $ are treated as environment variables for Linux. Does nothing if the environment variable is not found.

[slmgc]

Is there anyone who can review this small change? It's a real deal breaker for us 😔 @ehuss