do not run CI on nightly compiler

- failed CI because of ICE is annoying

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [stable, nightly]
+        rust: [stable]
         # For reference: https://github.com/actions/virtual-environments#available-environments
         os: [ubuntu-latest, windows-latest, macos-latest]
     steps:
@@ -48,7 +48,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [stable, nightly]
+        rust: [stable]
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1