ci: pin Rust toolchain to 1.84.0 to match upstream

Author: shefeek-jinnah

.github/workflows/ci.yml

@@ -20,6 +20,7 @@ jobs:
             - uses: actions/checkout@v4
             - uses: dtolnay/rust-toolchain@stable
               with:
+                  toolchain: "1.84.0"
                   components: clippy, rustfmt
             - uses: Swatinem/rust-cache@v2