Replace USER with BUILD_USERNAME

USER may be "nobody" if the build is sandboxed. BUILD_USERNAME will have
the original value in all cases.

Bug: 122270019
Test: none
Change-Id: Id7122113f1764cb2e676e96e255f47675f831f83
diff --git a/version.mk b/version.mk
index ed67dee..7ab307b 100644
--- a/version.mk
+++ b/version.mk
@@ -109,7 +109,7 @@
 ifeq ($(strip $(HAS_BUILD_NUMBER)),false)
   git_hash := $(shell git --git-dir $(LOCAL_PATH)/.git log -n 1 --pretty=format:%h)
   date_string := $$(date +%m%d%y_%H%M%S)
-  version_name_package := $(base_version_major).$(base_version_minor).$(base_version_build) (eng.$(USER).$(git_hash).$(date_string)-$(base_version_arch)$(base_version_density))
+  version_name_package := $(base_version_major).$(base_version_minor).$(base_version_build) (eng.$(BUILD_USERNAME).$(git_hash).$(date_string)-$(base_version_arch)$(base_version_density))
 else
   version_name_package := $(base_version_major).$(base_version_minor).$(base_version_build) ($(BUILD_NUMBER_FROM_FILE)-$(base_version_arch)$(base_version_density))
 endif