rust_uprev: fetch distfiles from local mirror

This adds a few extra steps to rust_uprev where rust sources and
rust-bootstrap prebuilts are fetched from the local mirror. This
ensures that these files exist and are readable, and that the
local versions (that will be used to compute the digests for the
manifest) match the ones on the mirror.

BUG=None
TEST=Ran the script with existing and non-existing rust versions

Change-Id: I9868b027356cf14fc10d9e56665c74ad26555345
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/3182632
Tested-by: Bob Haarman <inglorion@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
Commit-Queue: Bob Haarman <inglorion@chromium.org>
2 files changed