[CI] Use a lockfile for installing the `datadog` package Without a lockfile, it could fail to compile when the dependencies have changed. Reported [here](https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/CI.20failure.20in.20DataDog.20upload). r? `@jdno` try-job: x86_64-msvc-ext2