Make null builds always be null builds.

Previously, soong.environment.used was written after build.ninja and if
the amount of time that passed between the two was long enough, Ninja
would decide that build.ninja is older than soong.environment.used and
rebuild it.

Test: test_null_build in bootstrap_test.sh in a loop.
Change-Id: I5467da487e8e8f2646644b8a7fb9549b9ff18276