copies-rust: remove an unnecessary format!() inside assert!()
The `assert!()` macro supports formatting. Rust 2021 no longer
supports an unnecessary `format!()` inside it. I noticed because I
recently update my Rust toolchain and `test-check-cargo-lock.t`
started failing.
Differential Revision: https://phab.mercurial-scm.org/D10085
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)