ci(native): add ref to setup-pixi

To address:

> Error
> the `uses' attribute must be a path, a Docker image, or owner/repo@ref

.github/workflows/native.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-22.04
 
     steps:
-      - uses: prefix-dev/setup-pixi
+      - uses: prefix-dev/setup-pixi@v0.8.1
 
       - name: Build and Test ITK-Wasm
         run: |