Add a way to manually run build script

[Keruspe]

While updating some build.rs which performs code generation, it would be great to be able to just run it without a full build.

cargo run-build-script and/or a way to get the build script binary from cargo metadata would be greatly appreciated

[ehuss]

Thanks for the request! I think this is essentially a duplicate of #7178, so I'm going to close in favor of that.