Setup explicit Swift version in lint job

Author: mattt

.github/workflows/ci.yml

@@ -26,6 +26,11 @@ jobs:
       steps:
         - uses: actions/checkout@v4
 
+        - name: Set up Swift
+          uses: swift-actions/setup-swift@v2
+          with:
+            swift-version: "6.1"
+
         - run: |
             swift format lint --strict --recursive .