chore: use stable version of Rust when testing.

So that CI doesn't fail so often with brand new lints I've never
encountered before.

.github/workflows/rust.yml

@@ -13,10 +13,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v1
-    - name: Use Rust nightly
+    - name: Use Rust stable
       uses: actions-rs/toolchain@v1
       with:
-        toolchain: nightly
+        toolchain: stable
         components: clippy
         override: true
     - name: Build