fix some moved code from revert of aosp/2518976

aosp/2768553 reverted aosp/2518976, but there was some code that was
moved in between these two commits. This commit removes a code block
for some envvars that were moved in aosp/2638675, and adds back the code
for the ANDROID_RUST_DARWIN envvar.

Change-Id: I9ac09c608fe64340535ac493ab7f07ecb9b9c335