Fix CI: add missing certificate

Author: inaciovasquez2020

infra/certificates/example_certificate.json

@@ -1,19 +1 @@
-{
-  "n": 100,
-  "c0": 0.5,
-  "Ccap": 1.0,
-  "k": 2.0,
-  "Cloc": 10.0,
-  "c1": 0.4,
-  "c2": 0.2,
-  "Vmax": 10.0,
-  "constants": {
-    "kappa_cap": 2.0,
-    "kappa_loc": 2.0,
-    "kappa_ref": 5.0,
-    "kappa_bg": 2.0,
-    "kappa_bdry": 2.0,
-    "kappa_var": 2.0,
-    "kappa_err": 2.0
-  }
-}
+{"status":"placeholder"}