build_packages: Don't transitively upgrade  dev-lang/rust

Rust is getting upgraded in some builders because of transitive BDEPEND dependencies
causing build breakage. Exempt rust from building as a critical toolchain package
if a prebuilt is not available.

BUG=chromium:1050752, chromium:1126288

Change-Id: I399651e7b4bb4febabad64878348d39a670c63f2
Commit-Queue: Manoj Gupta <>
Tested-by: Manoj Gupta <>
Reviewed-by: Wei-Cheng Xiao <>
Reviewed-by: Mike Frysinger <>
diff --git a/build_packages b/build_packages
index 6a9f5f9..dfda48d 100755
--- a/build_packages
+++ b/build_packages
@@ -441,6 +441,7 @@
 # to `emerge` below once we figure out how to exclude toolchain packages from
 # being upgraded transitively via BDEPEND relations.
+  "dev-lang/rust"