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
TEST=`build_packages`

Change-Id: I399651e7b4bb4febabad64878348d39a670c63f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2399768
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Wei-Cheng Xiao <garryxiao@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed