fix: avoid accidental mutation in callback handling

This would mutate `affect`, so that if the same callback was `complete`d
multiple times, the equations would be appended multiple times.

Author: AayushSabharwal

lib/ModelingToolkitBase/src/systems/callbacks.jl

@@ -140,7 +140,7 @@ function AffectSystem(
         iv = t_nounits
         @warn "No independent variable specified. Defaulting to t_nounits."
     end
-    append!(affect, extra_eqs)
+    affect = [affect; extra_eqs]
 
     discrete_parameters = SymbolicAffect(affect; discrete_parameters).discrete_parameters