LOG_DIR changes should not trigger soong_build to re-run

On the build servers, LOG_DIR is a subset of DIST_DIR, which encodes the
build number. This means that LOG_DIR is guaranteed to be different for
every build, and we'll never re-use previous soong runs on CI builds.

The current use case (writing out soong metrics) shouldn't care if the
value changes, so pipe the environment variable through, bypassing the
env dep checks. It's not clear if this is the only issue triggering
soong_build re-runs on the build servers, but it's a start.

Test: Change DIST_DIR, soong_build does not re-run.
Change-Id: I59bf117f3fbc0e7f10f0506d3d1ac7564c6a00dc