fix: add Swift setup step for Linux runners

Author: coenttb

.github/workflows/swift-format.yml

@@ -1,4 +1,4 @@
-name: Format
+name: Swift Format
 
 on:
   push:
@@ -18,6 +18,11 @@ jobs:
     steps:
       - uses: actions/checkout@v5
 
+      - name: Setup Swift
+        uses: swift-actions/setup-swift@v2
+        with:
+          swift-version: '6.0'
+
       - name: Cache swift-format
         id: cache-swift-format
         uses: actions/cache@v4