CI: Fix nightly build

```
error: lint `suspicious_auto_trait_impls` has been removed: no longer needed, see #93367 <rust-lang/rust#93367> for more information
```

.github/workflows/build_docs.yaml

@@ -13,9 +13,8 @@ jobs:
  env:
  # Allow deprecated warning because we are using nightly and some things might be deprecated in nightly
  # for which the stable alternative is not yet available.
- # Allow suspicious-auto-trait-impls to work around https:/Diggsey/scoped-tls-hkt/issues/2
  # RUSTFLAGS: -D warnings -W deprecated
- RUSTDOCFLAGS: --html-in-header=/home/runner/work/slint/slint/docs/resources/slint-docs-highlight.html -D warnings -W deprecated -W suspicious-auto-trait-impls
+ RUSTDOCFLAGS: --html-in-header=/home/runner/work/slint/slint/docs/resources/slint-docs-highlight.html -D warnings -W deprecated
  SLINT_NO_QT: 1
  CARGO_INCREMENTAL: false
  steps: