ProcessManager.run signature is inconsistent with LocalProcessManager.run about process.dart

[jamesderlin]

This also makes it harder to migrate code that uses dart:io's Process.run/Process.start (which also expect nullable workingDirectory and environment) to using ProcessManager: calling code would need to explicitly pass Platform.environment and the current working directory to get the same behavior as passing null directory to dart:io's Process functions.

cc @jonahwilliams