MNT: Skip existing wheels during nightly wheel upload

Add the '--skip-existing' option for anaconda upload to

> Skip errors on package batch upload if it already exists

This will keep the nightlies.yml workflow from erroring out if
no new wheels were generated, or if a workflow is manually rerun.

Co-authored-by: Elliott Sales de Andrade <[email protected]>

Author: matthewfeickert

.github/workflows/nightlies.yml

@@ -61,4 +61,5 @@ jobs:
         run: |
           anaconda --token ${{ secrets.ANACONDA_ORG_UPLOAD_TOKEN }} upload \
             --user scipy-wheels-nightly \
+            --skip-existing \
             dist/matplotlib-*.whl