LLVM LIT: Is it possible to pass user defined parameter into a test script?

Do somebody know how to parametrize a llvm-lit script? I need to pass some environment variable value into a script to use it inside command line of some tool.

For example, test script could have the following line:

// RUN: some-tool $SOME_ENV_VAR 

I need to pass SOME_ENV_VAR value somehow.

According to the documentation for llvm-lit there is an option -D or --param, but is is unclear how to access this user defined parameter inside a script.

Thank you in advance.