github actions: nightly only

.github/workflows/rust.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macOS-latest]
-        rust: [nightly, beta, stable]
+        rust: [nightly]
 
     steps:
     - uses: actions/checkout@master
@@ -27,6 +27,7 @@ jobs:
 
     - name: check
       uses: actions-rs/cargo@v1
+      if: matrix.rust == 'nightly'
       with:
         command:  check
         args: --all --bins --examples --tests