Use develop workflow instead of test workflow for tests badge (#676)

The develop workflow is run on merges to main, which is really what we want to
track in this badge, rather than arbitrary runs from PRs that might be failing,
which is what we're getting now.

README.rst

@@ -147,8 +147,8 @@ This project is licensed under the BSD 2-Clause License - see the
    :target: https://pypi.org/project/socs/
    :alt: PyPI Package
 
-.. |tests| image:: https://github.com/simonsobs/socs/actions/workflows/pytest.yml/badge.svg
-    :target: https://github.com/simonsobs/socs/actions/workflows/pytest.yml
+.. |tests| image:: https://github.com/simonsobs/socs/actions/workflows/develop.yml/badge.svg?branch=main
+    :target: https://github.com/simonsobs/socs/actions/workflows/develop.yml
     :alt: GitHub Workflow Status
 
 .. |versions| image:: https://img.shields.io/pypi/pyversions/socs