Make it possible to debug soong_ui.

This works by setting the SOONG_UI_DELVE= environment variable to the
port on which soong_ui should accept a Delve connection on.

This is achieved by reusing the Delve execution logic between soong_ui
and soong_build.

Test: Manual.
Change-Id: Id2c1d4b6faac1a4a3918c91030ce2239f7daf54f