Update deprecated GitHub Actions commands.

[ehuss]

The old method of using ::set-env and ::add-path have been deprecated, see https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/.

This includes a second commit to work around a GitHub Actions cache bug on macOS.

[ehuss]

Note that CI caching is not really working for any job that doesn't set SKIP_TESTS. I filed rust-lang/cargo#8762 with a description of the problem. This will be somewhat mitigated by rust-lang/cargo#8755 which should be on nightly (that will fix CI caching, but all the dependencies are still being built twice). Some workarounds would be:

• Add lto to [profile.bench].
• Don't use --release when running tests.

[kinnison]

Thanks for doing this. It's always nice when things change under us 👍