Rollup merge of #121978 - GuillaumeGomez:dylib-duplicated-path, r=bjorn3

Fix duplicated path in the "not found dylib" error

While working on the gcc backend, I couldn't figure out why I had this error:

```
error: couldn't load codegen backend /checkout/compiler/rustc_codegen_gcc/target/release/librustc_codegen_gcc.so/checkout/compiler/rustc_codegen_gcc/target/release/librustc_codegen_gcc.so: cannot open shared object file: No such file or directory
```

As you can see, the path is duplicated for some reason. After investigating a bit more, I realized that `libloading::Error::LoadLibraryExW` starts with the path of the not found dylib, making it appear twice in our error afterward (because we do render it like this: `{path}{err}`, and since the `err` starts with the path...).

Thanks to `````@bjorn3````` for linking me to rust-lang/rust#121392. :)

Author: matthiaskrgr