Use liblog.so implementation from prebuilts/ for chroot testing.

Test: test.py on device
Bug: 142935992
Change-Id: I16b311738cb7d226c83cce517ff805b564093113
diff --git a/tools/buildbot-build.sh b/tools/buildbot-build.sh
index 2b127c2..37abdd0 100755
--- a/tools/buildbot-build.sh
+++ b/tools/buildbot-build.sh
@@ -141,7 +141,7 @@
   # testing, we need to install an implementation of the libraries (and cannot
   # rely on the one already installed on the device, if the device is post R and
   # has it).
-  implementation_libs="libartpalette-system.so"
+  implementation_libs="libartpalette-system.so liblog.so"
   if [ -d prebuilts/runtime/mainline/platform/impl ]; then
     if [[ $TARGET_ARCH = arm* ]]; then
       arch32=arm