`pkgconfig` has been a alias to `pkg-config` since 2021-01-18, and has started being an error since 2022-09-24.
This commit makes recent nixpkgs versions work, but might make version older than January 2021 break (which is probably not an issue).
I was able to build a working musl binary with this change, by running
this command:
```
cargo build --release --target x86_64-unknown-linux-musl
```
Thanks to @arxanas for the tip.
