ci: pin nightly version to avoid upstream regression

Author: cpu

.github/workflows/test.yaml

@@ -271,6 +271,10 @@ jobs:
 
       - name: Install nightly rust toolchain
         uses: dtolnay/rust-toolchain@nightly
+        with:
+          # TODO(XXX): remove toolchain pinning once upstream issue is resolved:
+          # https://github.com/rust-lang/rust/issues/139715
+          toolchain: nightly-2025-03-25
 
       - name: Install cbindgen
         # Pin the installed version of cbindgen so that local usage can be