A better story for canary jobs (yellow CI)

[jonhoo]

This PR does two things.

First, it adds a new smoke test to the mix: cargo check -- -D warnings, which will error on any warning from rustc.

Second, it makes it easy to opt out of all canaries (basically anything that is allow_fail) by setting

parameters:
  canaries: false

This will currently disable:

• rustfmt on beta
• clippy on beta
• the new -D warnings test
• test on nightly

This parameter is intended for users that worry about long-running or highly parallel CI, yellow CI ("partially successful").