Merge #266

266: Make size-diff pass if building master fails. r=hudson-ayers a=jrvanwhy

size-diff has been painful in 2 cases:

1. If master is broken, it fails on PRs that fix master's build.
2. If the commands required to build the examples change, it breaks. This happened when we changed rust-toolchain to the TOML format, and removed the `rustup target add` commands from the size-diff workflow. This change was fine once it hit master but broke in the PR.

size-diff still fails if the PR's build fails, so we can still detect PRs that break size-diff's functionality.

Co-authored-by: Johnathan Van Why <jrvanwhy@google.com>