[ci] fix cargo hack test invocation

[sunshowers]

No description provided.

[sunshowers]

To address the compilation failures against Rust 1.56, one option is to:

• check in a Cargo.lock that represents a set of versions that builds against Rust 1.56.
• Run tests against that version
• Also on stable, delete the Cargo.lock and run tests against the latest versions with a fresh set of versions.

[sunshowers]

Looks like Cargo has https://doc.rust-lang.org/cargo/reference/unstable.html#direct-minimal-versions now as well.