Remove zeros from build number

The zeros were kept in place of the timestamp in case people were
parsing it. Let's try to remove the zeros now and see what breaks.

Test: Presubmits
Change-Id: Ic52168f6be05a74323d8e47935de5c6a5cf15516
diff --git a/ui/build/build.go b/ui/build/build.go
index 03d8392..b2d16fc 100644
--- a/ui/build/build.go
+++ b/ui/build/build.go
@@ -79,7 +79,7 @@
 		if username, ok = config.environ.Get("BUILD_USERNAME"); !ok {
 			ctx.Fatalln("Missing BUILD_USERNAME")
 		}
-		buildNumber = fmt.Sprintf("eng.%.6s.00000000.000000", username)
+		buildNumber = fmt.Sprintf("eng.%.6s", username)
 		writeValueIfChanged(ctx, config, config.OutDir(), "file_name_tag.txt", username)
 	}
 	// Write the build number to a file so it can be read back in