servo: mark broken

The current version broke in a Rust upgrade and newer versions fail to
build mozjs-sys, because our cargo fetcher leaves a Cargo.toml with
template vars around, which the mozjs-sys build process finds and
subsequently chokes on:

```
called `Result::unwrap()` on an `Err` value: CargoMetadata {
  stderr: "error: failed to get `encoding_c` as a dependency of package `mozjs_sys v0.140.0-3 (/build/servo-0-unstable-2025-10-04-vendor/mozjs_sys-0.140.0-3)`

Caused by:
  failed to load source for dependency `encoding_c`

Caused by:
  Unable to update registry `crates-io`

Caused by:
  failed to update replaced source registry `crates-io`

Caused by:
  failed to read root of directory source: /build/servo-0-unstable-2025-10-04-vendor/@vendor@

Caused by:
  No such file or directory (os error 2)""
```

pkgs/by-name/se/servo/package.nix

@@ -159,6 +159,7 @@ rustPlatform.buildRustPackage {
   };
 
   meta = {
+    broken = true; # cargo fetcher leaves invalid Cargo.toml around, which breaks mozjs-sys build
     description = "Embeddable, independent, memory-safe, modular, parallel web rendering engine";
     homepage = "https://servo.org";
     license = lib.licenses.mpl20;