Switch back to nightly toolchain

Now rust-lang/rust#83153 has been merged,
fixing rust-lang/rust#83126 .

Author: est31

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [macOS-latest, ubuntu-latest]
-        toolchain: [stable, beta, nightly-2021-02-23]
+        toolchain: [stable, beta, nightly]
 
     runs-on: ${{ matrix.os }}