CI: run fmt & docs on nightly for #[feature]

Author: Fishrock123

.github/workflows/ci.yaml

@@ -52,11 +52,11 @@ jobs:
     steps:
     - uses: actions/checkout@master
 
-    - name: Install stable toolchain
+    - name: Install nightly toolchain
       uses: actions-rs/toolchain@v1
       with:
         profile: minimal
-        toolchain: stable
+        toolchain: nightly
         override: true
         components: rustfmt