Bump build id to 20.12.0-beta.1

Change-Id: I9f472d5b0e5ce93fbe60a1b5184e83911a1dee9e
diff --git a/core/build_id.mk b/core/build_id.mk
index 9af6418..bd5ae67 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-beta.0
+export BUILD_ID=20.12.0-beta.1