Print free disk space when starting build_test.

This is so that we know whether the attached bug is caused by the disk
being full.

Bug: 187870338
Test: Presubmits.
Change-Id: Ib09f51d971de556e5737ad2754836f2a12f60281
diff --git a/build_test.bash b/build_test.bash
index 3230f2d..296a79c 100755
--- a/build_test.bash
+++ b/build_test.bash
@@ -49,6 +49,10 @@
 esac
 
 echo
+echo "Free disk space:"
+df -h
+
+echo
 echo "Running Bazel smoke test..."
 "${TOP}/tools/bazel" --batch --max_idle_secs=1 info