Enable paths with environment variables for local steps in the offline workflow editor

[atanas-bitrise]

When editing workflows with the offline workflow editor, local steps with paths that have environment variables cannot be found by the stepman tool. The workflow editor fails to load the workflow in that case and displays an error instead.

The workflow editor supports both absolute and relative paths. However, it doesn't support environment variables because the library that executes the stepman command doesn't support them by design. However, when the use case is narrowed down to local Steps that use the path:: prefix, it makes sense for environment variables to be supported.

This change adds a productivity boost for large teams that use their own Bitrise Steps locally.