Make size-diff pass if building master fails.
Make size-diff pass if building master fails.

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.

tock/libtock-rs