fix: enable user configuration of build_command env vars

[codejedi365]

Purpose

• Add the ability for the user to configure the set of environment variables to pass to the build command subprocess
• Resolve build_command environment variables not working after upgrade from 9.6.0 to 9.7.0 #922

Rationale

Primarily to be good stewards of the provided authentication tokens to PSR, we do not blindly pass the entire parent process environment on to the child process of the build. However, as #922 mentioned, there is a legitimate need to pass some user-defined environment variables on to the build_command child process. This happy median is to provide a build_command_env configuration option that will handle definitions of either hardcoded values or variables desired to be passed along to the subprocess.

How I tested

Added a separate cli test with a pyproject.toml configuration that included all the ways users can provide an environment variable. We allow direct pass through, and hardcoded variables (empty or not). I also ran the psr-test-gha repo with a build script that would of failed if it didn't receive the proper environment variables and it succeeded.