chore: standardize CI cache key formatting

Update cache restore-keys to use proper YAML multiline format with pipe character for consistency across all jobs. This aligns with CI standard formatting conventions.

Author: coenttb

.github/workflows/ci.yml

@@ -64,7 +64,8 @@ jobs:
         with:
           path: .build
           key: ${{ runner.os }}-spm-${{ hashFiles('Package.swift') }}
-          restore-keys: ${{ runner.os }}-spm-
+          restore-keys: |
+            ${{ runner.os }}-spm-
 
       # Note: swift test builds automatically in release mode
       - name: Test (release)
@@ -84,7 +85,8 @@ jobs:
         with:
           path: .build
           key: ${{ runner.os }}-swift60-spm-${{ hashFiles('Package.swift') }}
-          restore-keys: ${{ runner.os }}-swift60-spm-
+          restore-keys: |
+            ${{ runner.os }}-swift60-spm-
 
       # Note: swift test builds automatically
       - name: Test (Swift 6.0)