Reference latest Pixi version in README (#167)

Author: github-actions[bot]

README.md

@@ -25,7 +25,7 @@ GitHub Action to set up the [pixi](https://github.com/prefix-dev/pixi) package m
 ```yml
 - uses: prefix-dev/setup-pixi@v0.8.1
   with:
-    pixi-version: v0.39.5
+    pixi-version: v0.40.2
     cache: true
     auth-host: prefix.dev
     auth-token: ${{ secrets.PREFIX_DEV_TOKEN }}