[build] Fix integration with Zircon build when it uses ASan

This entailed another substantial cleanup of environment_redirect()
to handle more cases.  It also necessitated reorganizing the deps
in the Zircon build for the legacy integration.  The new arrangement
should be much easier to understand.

Bug: BLD-447 #comment Fixed --variant asan build
Test: fx set bringup.x64 --variant asan && fx build
Change-Id: I2cd8d7aa95eba5f34af421c75d634f31d988a1f8