rustPlatform.buildRustPackage: cleaner output on verifyCargoDeps (#77567)

When this fails, the user may want to copy-paste the path to the "bad"
Cargo.lock file to inspect. The trailing `.` on `$cargoDeps.` gets caught in
most terminal copy-pastes. Since half the lines already don't have it, this
removes it from all of them for consistent output.