buildRustCrate fails to build rustversion 1.0.2

[nagisa]

Describe the bug

buildRustCrate will clobber the source directory of the rustversion crate and it will subsequently fail to build with an error like this:

builder for '/nix/store/qzmcgmdyl3vhsj3y5gzfc1miqjxmddxn-rust_rustversion-1.0.2.drv' failed with exit code 1; last 10 log lines:
  Building src/lib.rs (rustversion)
  Running rustc --crate-name rustversion src/lib.rs --out-dir target/lib --emit=dep-info,link -L dependency=target/deps --cap-lints allow -C opt-level=3 -C codegen-units=16 --extern proc_macro2=/nix/store/p0aydc61mn5r4nw4l3c3zmpz1cv4dh4d-rust_proc-macro2-1.0.8-lib/lib/libproc_macro2-436ab8affb.rlib --extern quote=/nix/store/18vbqj8sj4dpnrr7dm19l2vaiail59y6-rust_quote-1.0.2-lib/lib/libquote-3ffc3269a3.rlib --extern syn=/nix/store/2mplqk67f8kfjnl1c2qnmy4md9sz2lf3-rust_syn-1.0.14-lib/lib/libsyn-121d03fbaa.rlib --cfg feature="default" --edition 2018 -C metadata=9fc8a6261b -C extra-filename=-9fc8a6261b --crate-type proc-macro -L /build/rustversion-1.0.2/target/build/rustversion.out --color always
  error: expected one of `!` or `::`, found `{`
   --> src/version.rs:1:9
    |
  1 | Version {
    |         ^ expected one of `!` or `::`

  error: aborting due to previous error

The reason this happens is because:

1. rustversion's build script generates $OUT_DIR/version.rs;
2. buildRustCrate copies it over the original src/version.rs.

To Reproduce
Steps to reproduce the behavior:

1. crate2nix a crate dependent on rustversion (or maybe rustversion itself?)
2. nix build

Expected behavior

The original source file should not be replaced. I don’t believe this copying code should exist at all.

cc @andir

[kolloch]

This might be a duplicate of #74071