Fix test-nightly CI

Forgot to install Pixi

Author: VeckoTheGecko

.github/workflows/upstream-dev-ci.yaml

@@ -62,7 +62,20 @@ jobs:
       - uses: actions/checkout@v5
         with:
           fetch-depth: 0 # Fetch all history for all branches and tags.
-
+      - name: Restore cached pixi lockfile
+        uses: actions/cache/restore@v4
+        id: restore-pixi-lock
+        with:
+          enableCrossOsArchive: true
+          path: |
+            pixi.lock
+          key: ${{ needs.cache-pixi-lock.outputs.cache-id }}
+      - uses: prefix-dev/setup-pixi@v0.9.0
+        with:
+          pixi-version: ${{ env.PIXI_VERSION }}
+          cache: true
+          environments: ${{ matrix.pixi-env }}
+          cache-write: ${{ github.event_name == 'push' && github.ref_name == 'main' }}
       - name: Version info
         run: |
           pixi run -e ${{matrix.pixi-env}} python xarray/util/print_versions.py