disable shareable_constant_value for CI

To debug CI failures on FreeBSD, disable `shareable_constant_value`.

lib/time.rb

@@ -480,10 +480,17 @@ def strptime(date, format, now=self.now)
       t
     end
 
-    MonthValue = { # :nodoc:
+# TODO: CI failure on FreeBSD
+#       http://rubyci.s3.amazonaws.com/freebsd12/ruby-master/log/20210425T203001Z.fail.html.gz
+# shareable_constant_value: none
+
+    MonthValue = Ractor.make_shareable({ # :nodoc:
       'JAN' => 1, 'FEB' => 2, 'MAR' => 3, 'APR' => 4, 'MAY' => 5, 'JUN' => 6,
       'JUL' => 7, 'AUG' => 8, 'SEP' => 9, 'OCT' =>10, 'NOV' =>11, 'DEC' =>12
-    }
+    })
+
+# shareable_constant_value: literal
+
 
     #
     # Parses +date+ as date-time defined by RFC 2822 and converts it to a Time