buildRustPackage: Fix "warning: file ... may be generated" (#24471)

Every Rust derivation used to emit a warning like the following:

```
setting SOURCE_DATE_EPOCH to timestamp 1490877042 of file cargo-6e0c18c/Cargo.lock
warning: file cargo-6e0c18c/Cargo.lock may be generated; SOURCE_DATE_EPOCH may be non-deterministic
```

The reason is that the dependencies are copied without preserving
timestamps. Changing the build script to timestamp-preserving copy
removes the warning.