ci,cgen: put the Interface(unsafe{nil}) change for now behind -experimental (#21593)

Author: spytheman

vlib/v/gen/c/cgen.v

@@ -2564,12 +2564,14 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type_raw ast.Type, expected_typ
 			if exp_sym.info.is_generic {
 				fname = g.generic_fn_name(exp_sym.info.concrete_types, fname)
 			}
-			// Do not allocate for `Interface(unsafe{nil})` casts
-			is_nil_cast := expr is ast.UnsafeExpr && expr.expr is ast.Nil
-			if is_nil_cast {
-				g.write('/*nili*/')
-				g.write('((void*)0)')
-				return
+			if g.pref.experimental {
+				// Do not allocate for `Interface(unsafe{nil})` casts
+				is_nil_cast := expr is ast.UnsafeExpr && expr.expr is ast.Nil
+				if is_nil_cast {
+					g.write('/*nili*/')
+					g.write('((void*)0)')
+					return
+				}
 			}
 			g.call_cfn_for_casting_expr(fname, expr, expected_is_ptr, exp_styp, got_is_ptr,
 				false, got_styp)