Build only library target on Linux CI

The SwiftDOF_E2E executable uses Apple-only APIs (NSKeyValueObservation,
URLSession) that are not available on Linux.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: RISCfuture

.github/workflows/ci.yml

@@ -47,7 +47,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Build
-        run: swift build -v
+        run: swift build --target SwiftDOF -v
       - name: Test
         run: swift test -v
   lint: