CI: Update ci-sage.yml to use sagemath/sage#36276

Author: mkoeppe

.github/workflows/ci-sage.yml

@@ -102,8 +102,9 @@ jobs:
       targets:                  SAGE_CHECK=no SAGE_CHECK_PACKAGES=sympy build SAGE_DOCTEST_RANDOM_SEED=0 ptest
       # Standard setting: Test the current beta release of Sage.
       sage_repo:                sagemath/sage
-      sage_ref:                 develop
+      # sage_ref:                 develop
       # To test with a Sage PR, use this:
       #sage_ref:                 refs/pull/PR_NUMBER/merge
+      sage_ref:                 refs/pull/36276/merge
       upstream_artifact:        upstream
     needs: [dist]