CI: use nightly toolchain by default

Author: fitzgen

.github/workflows/rust.yml

@@ -10,6 +10,11 @@ jobs:
     steps:
     - uses: actions/checkout@v1
 
+    - name: Install nightly Rust
+      run: |
+        rustup toolchain install nightly
+        rustup default nightly
+
     - name: Check rustfmt
       run: |
         rustup component add rustfmt --toolchain stable

rust-toolchain

@@ -0,0 +1 @@
+nightly