feat(snix/rust-docs): fail build if there's broken references

Thanks to cl/30305, we don't have broken references in the rust docs anymore.

Enforce for it to stay this way, by making the //snix:rust-docs target
fail if there's broken references.

References to private items (which we still have) are logged as
warnings.

Change-Id: I06e6fe3bec3d0c4e6af910685f9cd0650e67dc5c
Reviewed-on: https://cl.snix.dev/c/snix/+/30312
Tested-by: besadii
Autosubmit: Florian Klink <flokli@flokli.de>
Reviewed-by: Bence Nemes <nemes.bence1@gmail.com>

snix/default.nix

@@ -82,7 +82,7 @@ in
     ] ++ lib.optional pkgs.stdenv.isDarwin pkgs.libiconv;
 
     buildPhase = ''
-      cargo doc --document-private-items
+      RUSTDOCFLAGS="-D rustdoc::broken-intra-doc-links" cargo doc --document-private-items
       mv target/doc $out
     '';
   };