Use matrix for Linux tests

Author: finestructure

.github/workflows/ci.yml

@@ -28,8 +28,14 @@ jobs:
   Linux:
     # https://github.com/actions/runner-images
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        image: [
+          'swift:5.8',
+          'swiftlang/swift@sha256:f212429011a4c3592ffaa660fd78b5bc149e9df7920da02dfc0e7a761582fa10' # swiftlang/swift:nightly-5.9-focal 2023-08-16
+        ]
     container:
-      image: swiftlang/swift@sha256:f212429011a4c3592ffaa660fd78b5bc149e9df7920da02dfc0e7a761582fa10  # swiftlang/swift:nightly-5.9-focal 2023-08-16
+      image: ${{ matrix.image }}
     steps:
       - uses: actions/checkout@v3
       - run: git config --global user.email "[email protected]"