Update dependency bazelbuild/bazel-skylib to latest version

Included changes:

  - 56b235e700ddd6a15b7d9fa1803fa7a84048471e README.md: fix broken directory rule links (#544)
  - e8e9d218ff563f93e3a7f7547ea532e66b210620 Explicitly enable workspace for some tasks (#540)
  - 01244de9b0789a4ce8fdb8182a6e1482cbc3e330 Fix bazel_skylib with Bazel@HEAD (#538)

BAZEL_SKYLIB_VERSION_REV_ID: 56b235e700ddd6a15b7d9fa1803fa7a84048471e
Change-Id: I0359a44b451770a6ab7d6d6760fd46c111675d96

Author: Copybara Team

third_party/bazel_skylib.bzl

@@ -15,5 +15,5 @@
 
 # This file is autogenerated by copybara, please do not edit.
 
-skylib_version = "5c071b5006bb9799981d04d74a28bdee2f000d4a"
-skylib_sha256 = "812075804e708462e7b3cb9d31c2742c049980b600fe044cc6921d925ce76867"
+skylib_version = "56b235e700ddd6a15b7d9fa1803fa7a84048471e"
+skylib_sha256 = "5e6c9af21b6d6bc5cc32feef0253060c42331a2c65241f5e1b83a662b9fd7b65"