Allow not buffering the output of soong_build .

This behavior is enabled by setting the environment variable
SOONG_UNBUFFERED_OUTPUT=1 .

This is not the default behavior because only one process can be running
in the console pool and this would thus result in a loss of parallelism
if multiple soong_build invocations are needed.

Changing the value of this environment variable does not result in a
rebuild, since it does not affect the build outputs.

Test: Presubmits.
Change-Id: I49d15eb145966bd805f0a47b46866285203b122a