Use pixi exec in autobump workflow (#8)

Author: pavelzw

template/.github/workflows/{% if add_autobump_workflow %}update-lockfiles.yml{% endif %}

@@ -19,8 +19,7 @@ jobs:
           run-install: false
       - name: Update lockfiles
         run: |
-          pixi global install pixi-diff-to-markdown
-          pixi update --json --no-install | pixi-diff-to-markdown >> diff.md
+          pixi update --json --no-install | pixi exec pixi-diff-to-markdown >> diff.md
       - name: Create pull request
         uses: peter-evans/create-pull-request@c5a7806660adbe173f04e3e038b0ccdcd758773c
         with: