workflows: nightly: use proxy URL for indexing

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>

Author: threexc

.github/workflows/nightly.yml

@@ -15,7 +15,7 @@ permissions:
   issues: read
 
 env:
-  PIP_EXTRA_INDEX_URL: https://gitlab.com/api/v4/projects/56254198/packages/pypi/simple
+  PIP_EXTRA_INDEX_URL: https://pypi.riseproject.dev/simple/
 
 jobs:
   check_versions: