[ci] Permit skipping `cargo-semver-checks` for a particular PR

[joshlf]

Sometimes cargo-semver-checks has false positives. It's not desirable, in these cases, to simply use the GitHub web UI "Merge without waiting for requirements to be met" option:

The reason is that this option also bypasses the merge queue, which we use to perform a number of checks which are disabled during a normal PR CI run.

Instead, we should teach the cargo-semver-checks CI step to check for an explicit directive in a PR's last commit message which instructs it to skip performing its checks. Something like:

SKIP_CARGO_SEMVER_CHECKS=1

Commit message

One important detail is that this directive should go in the last (ie most recent) commit message in the PR. Most of our PRs contain a single commit, but sometimes they contain multiple commits (e.g. when we use the web UI to merge recent changes from the target branch), and third-party contributors often don't use our development flow, and include multiple commits. The reason to use the commit message (as opposed to, for example, the PR message) is that:

• It's easier to check for in a GitHub Action
• It will be trivially available when the PR is running through the merge queue - by that time, multiple commits have been squashed, and their commit messages have been merged
• It's a minor inconvenience to users who get this wrong - CI will fail, and so they'll know that they need to fix something
• It will not admit false positives (ie, we'll never incorrectly skip cargo-semver-checks)