Don't use export with BUILD_ID

It's not necessary, and all the tools that read this file have been
updated.

Bug: 73959648
Test: cs for uses
Change-Id: Ic203e83ab437beecf07760660f2b3ac7c67e9ded
diff --git a/core/build_id.mk b/core/build_id.mk
index 0e8e2cf..2f9c3f3 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=MASTER
+BUILD_ID=MASTER