Eliminate dependency on unmaintained node12-based action

Reviewed By: diliop

Differential Revision: D47220090

fbshipit-source-id: 0b69dcce8115db028d194746fb1fc4fa2d283ad4

Author: David Tolnay

.github/workflows/ci.yml

@@ -15,10 +15,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - uses: actions-rs/toolchain@v1
+    - uses: dtolnay/rust-toolchain@nightly
       with:
-        toolchain: nightly
-        override: true
         components: clippy, rustfmt
     # We use rustfmt 2.0 for formatting, which differs from the released
     # version installed by Cargo