ci: Switch from deprecated actions-rs/toolchain to dtolnay/rust-toolchain

actions-rs/toolchain is archived/deprecated. dtolnay/rust-toolchain is
the actively maintained replacement.

Assisted-by: OpenCode (Claude claude-opus-4-6)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

.github/workflows/ci.yml

@@ -21,9 +21,7 @@ jobs:
     runs-on: ubuntu-24.04
     timeout-minutes: 15
     steps:
-    - uses: actions-rs/toolchain@v1
-      with:
-        toolchain: nightly
+    - uses: dtolnay/rust-toolchain@nightly
     - uses: actions/checkout@v4
     - run: cargo +nightly -Z package-workspace package