munin: fix build

fallout from perl 5.24 -> 5.28 update

pkgs/servers/monitoring/munin/default.nix

···
     perlPackages.IOStringy
   ];
 
+  # needs to find a local perl module during build
+  PERL_USE_UNSAFE_INC = stdenv.lib.optionalString (stdenv.lib.versionAtLeast (stdenv.lib.getVersion perl) "5.26") "1";
+
   # TODO: tests are failing http://munin-monitoring.org/ticket/1390#comment:1
   # NOTE: important, test command always exits with 0, think of a way to abort the build once tests pass
   doCheck = false;