Add Xcode 13 and Linux Swift 5.5.0 test builds

...

Author: helje5

.github/workflows/swift.yml

@@ -16,6 +16,7 @@ jobs:
           - swift:5.2.5-xenial
           - swift:5.3.2-xenial
           - swift:5.3.2-bionic
+          - swift:5.5.0-focal
     container: ${{ matrix.image }}
     steps:
       - name: Checkout Repository
@@ -30,7 +31,7 @@ jobs:
       - name: Select latest available Xcode
         uses: maxim-lobanov/setup-xcode@v1.2.1
         with:
-          xcode-version: 12.2
+          xcode-version: 13
       - name: Checkout Repository
         uses: actions/checkout@v2
       - name: Build Swift Debug Package