CI: Deny warnings in all CI jobs by default

[briansmith]

Currently the clippy GitHub Actions job is the only thing that is run with --deny warnings. But, that only runs for the x86-64-unkown-linux-gnu target. Instead, every CI job should have --deny warnings.

The Rust toolchain doesn't make this super easy; see rust-lang/cargo#8424. Warnings that only occur in other configurations are not denied in CI.

Proposal:

• Step 1: Change mk/clippy.sh: If $CI is NOT set, keep the default behavior. IF $CI is set as it is in GitHub Actions, ADD (don't replace) --deny warnings into $RUSTFLAGS. This will improve the situation
• Step 2 (A separate PR, ideally): Create a mk/clippy.ps1 that is the Windows (non-MinGW) counterpart to mk/clippy.sh, change all the GitHub Actions workflows to use it, and add the same logic w.r.t. %CI% to it.

See actions/runner#374 for info on how Github Actions sets $CI. I think other environments do similar.