Bump build id to 20.12.0-beta.0

Change-Id: I769e6833ed25912a98c5d428969ca736682d2398
diff --git a/core/build_id.mk b/core/build_id.mk
index 5520ebf..9af6418 100644
--- a/core/build_id.mk
+++ b/core/build_id.mk
@@ -18,4 +18,4 @@
 # (like "CRB01").  It must be a single word, and is
 # capitalized by convention.
 
-export BUILD_ID=20.12.0
+export BUILD_ID=20.12.0-beta.0