Update CI settings

.github/workflows/rust.yml

@@ -1,13 +1,25 @@
 name: Rust
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request: {}
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v2
-    - name: Build
-      run: cargo build --verbose
-    - name: Run tests
-      run: cargo test --verbose
+    - uses: actions-rs/cargo@v1
+      with:
+        command: test
+
+  check-format:
+    runs-on: ubuntu-18.04
+    steps:
+    - uses: actions/checkout@v1
+    - uses: actions-rs/cargo@v1
+      with:
+        command: fmt
+        args: -- --check