Make GENERATE_BAZEL_FILES=true correct.

This is achieved by writing soong.environment.used in Main() instead of
as a side effect of a singleton. This makes a difference because build
actions are not generated when GENERATE_BAZEL_FILES=true is set,
therefore the side effect did not happen.

Arguably, Main() is made worse by this change, but I don't want to
tackle the problem of readably determining which mode soong_build is
running in in this change.

Test: Presubmits + the additional test.
Change-Id: I66af2429aedf008762173eaaa55b828b4cf4328b