Prepare github actions CI for use by homu.

.github/workflows/rust.yml

@@ -2,21 +2,34 @@ name: Rust
 
 on:
   push:
-    branches: [ master ]
+    branches: ["**"]
   pull_request:
-    branches: [ master ]
+    branches: ["**"]
 
 env:
   CARGO_TERM_COLOR: always
 
 jobs:
   build:
-
-    runs-on: macos-11.0
-
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-11.0, macos-10.15]
     steps:
     - uses: actions/checkout@v2
     - name: Build
       run: cargo build --verbose
     - name: Run tests
       run: cargo test --verbose
+  build_result:
+    name: homu build finished
+    runs-on: ubuntu-latest
+    needs:
+      - "build"
+    steps:
+      - name: Mark the job as successful
+        run: exit 0
+        if: success()
+      - name: Mark the job as unsuccessful
+        run: exit 1
+        if: "!success()"