Downgrade rust toolchain.

Author: emilio

.github/workflows/bindgen.yml

@@ -23,7 +23,7 @@ jobs:
         with:
           # TODO: Should ideally be stable, but we use some nightly-only
           # features.
-          toolchain: nightly
+          toolchain: nightly-2025-04-01
           components: rustfmt, clippy
 
       - name: Run rustfmt