ci(windows): reduce cache restore time by caching registry only

- Restrict Swatinem/rust-cache to Windows only (Linux now uses Docker
  with Blacksmith layer caching)
- Set cache-targets: false to skip the 186MB target directory cache,
  reducing download from ~186MB to registry-only (~20-30MB)
- Network bandwidth was the bottleneck: cache restore took longer than
  the actual build on the Windows runner

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pythoninthegrass

.github/actions/setup-tauri-build/action.yml

@@ -147,10 +147,11 @@ runs:
         Add-MpPreference -ExclusionPath "$env:GITHUB_WORKSPACE"
         Add-MpPreference -ExclusionPath "$env:GITHUB_WORKSPACE\target"
 
-    - name: Cargo cache (Linux/Windows runners)
-      if: runner.os == 'Linux' || runner.os == 'Windows'
+    - name: Cargo registry cache (Windows)
+      if: runner.os == 'Windows'
       uses: Swatinem/rust-cache@v2
       with:
         cache-on-failure: "true"
+        cache-targets: "false"
       env:
         CARGO_INCREMENTAL: "0"