`build-std` doesn't work

We want to use the `-Zbuild-std` cargo flag to rebuild the standard library sanitized. This practice is requiredy by various sanitizers to work properly, for instance ThreadSanitizer and MemorySanitizer.

Since `build-std` is a Cargo flag it cannot be passed via `extra_rustc_flags`. So I tried adding a Cargo config file like

```toml
[unstable]
build-std = ["core", "std", "alloc", "proc_macro"]
```

to the `cargo_config` in `crates_repository`. That doesn't seem to have any effect at all though. Bug?

AFAICS to get behavior like with `build-std` we'd have to use a override the default BUILD file for the standard library, but that doesn't seem like a nice solution considering that this should work with a flag in the cargo config file.

Maybe I'm overlooking something though and this is already supported somehow?

Original issue: https://github.com/allada/turbo-cache/issues/188#issuecomment-1637103453