cros_setup_toolchains: generate prebuilts for rust-bootstrap 1.47.0

...mostly so we can pull these down and cache them, so upgrading to rust
1.50 is doable.

BUG=chromium:1185882
TEST=sdk tryjob

Change-Id: Ica8f9af767f0399bec0cebfd472757c4f8e9b500
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2774612
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
diff --git a/scripts/cros_setup_toolchains.py b/scripts/cros_setup_toolchains.py
index 57ebbbb..3a1d077 100644
--- a/scripts/cros_setup_toolchains.py
+++ b/scripts/cros_setup_toolchains.py
@@ -74,6 +74,7 @@
 HOST_POST_CROSS_PACKAGES = (
     'dev-lang/rust',
     'dev-lang/rust-bootstrap:0',
+    'dev-lang/rust-bootstrap:1.47.0',
     'virtual/target-sdk-post-cross',
     'dev-embedded/coreboot-sdk',
 )
@@ -82,7 +83,9 @@
 # bot hasn't had a chance to run yet, there are no binary packages available,
 # so we have to list them here and wait.  Once it completes, entries here can
 # be removed so they'll end up on bots & dev's systems.
-NEW_PACKAGES = ()
+NEW_PACKAGES = (
+    'dev-lang/rust-bootstrap:1.47.0',
+)
 
 # Enable the Go compiler for these targets.
 TARGET_GO_ENABLED = (