Build on macOS Big Sur with GitHub Actions (#132)

Now that Big Sur is available on GitHub Actions, we can get more certainty that everything works with the new OS (on Intel hardware for now).

* Build on macOS Big Sur with GitHub Actions
* Clarify Ubuntu 18.04 job name

.github/workflows/swift.yml

@@ -7,7 +7,7 @@ on:
     branches: [main]
 
 jobs:
-  macos-swift5_2:
+  catalina-swift5_2:
     runs-on: macos-10.15
 
     steps:
@@ -19,10 +19,21 @@ jobs:
           brew bundle
           cd TestApp && ../.build/debug/carton test
           ../.build/debug/carton bundle
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+  
+  bigsur-swift5_2:
+    runs-on: macos-11.0
 
-  macos-swift5_3:
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build on macOS 11.0 with Swift 5.2
+        run: |
+          sudo xcode-select --switch /Applications/Xcode_11.7.app/Contents/Developer
+          swift build
+          brew bundle
+          cd TestApp && ../.build/debug/carton test
+          ../.build/debug/carton bundle
+
+  catalina-swift5_3:
     runs-on: macos-10.15
 
     steps:
@@ -34,10 +45,21 @@ jobs:
           brew bundle
           cd TestApp && ../.build/debug/carton test
           ../.build/debug/carton bundle
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
-  linux-swift5_3:
+  bigsur-swift5_3:
+    runs-on: macos-11.0
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build on macOS 11.0 with Swift 5.3
+        run: |
+          sudo xcode-select --switch /Applications/Xcode_12.app/Contents/Developer
+          swift build
+          brew bundle
+          cd TestApp && ../.build/debug/carton test
+          ../.build/debug/carton bundle
+          
+  ubuntu18_04-swift5_3:
     runs-on: ubuntu-18.04
 
     steps: