Update the version of skylib bundled in the distfile

To support building bazel offline just out of the distribution archive,
all dependencies that would normally be downloaded during the build
are bundled in the distribution archive. So when updating skylib, the
@addition_distfiles repository needs to be updated as well. Do so now.

To be cherry-picked for #6493, i.e., the bazel 0.21.0 release.

Change-Id: I6bf813324298a9b2ca3cb2cac15a871eef9e46f9
PiperOrigin-RevId: 224323158

WORKSPACE

@@ -97,18 +97,19 @@ load("//:distdir.bzl", "distdir_tar")
 
 distdir_tar(
     name = "additional_distfiles",
+    # Keep in sync with the archives fetched as part of building bazel
     archives = [
         "fd937f4180c1b557805219af4482f1a27eb0ff2b.zip",
-        "7490380c6bbf9a5a060df78dc2222e7de6ffae5c.tar.gz",
+        "f83cb8dd6f5658bc574ccd873e25197055265d1c.tar.gz",
     ],
     dirname = "derived/distdir",
     sha256 = {
         "fd937f4180c1b557805219af4482f1a27eb0ff2b.zip": "43b8fcc56a180e178d498f375fbeb95e8b65b9bf6c2da91ae3ae0332521a1a12",
-        "7490380c6bbf9a5a060df78dc2222e7de6ffae5c.tar.gz": "3528fc6012a78da6291c00854373ea43f7f8b6c4046320be5f0884f5b3385b14",
+        "f83cb8dd6f5658bc574ccd873e25197055265d1c.tar.gz": "ba5d15ca230efca96320085d8e4d58da826d1f81b444ef8afccd8b23e0799b52",
     },
     urls = {
         "fd937f4180c1b557805219af4482f1a27eb0ff2b.zip": ["https://github.com/google/desugar_jdk_libs/archive/fd937f4180c1b557805219af4482f1a27eb0ff2b.zip"],
-        "7490380c6bbf9a5a060df78dc2222e7de6ffae5c.tar.gz": ["https://github.com/bazelbuild/bazel-skylib/archive/7490380c6bbf9a5a060df78dc2222e7de6ffae5c.tar.gz"],
+        "f83cb8dd6f5658bc574ccd873e25197055265d1c.tar.gz": ["https://github.com/bazelbuild/bazel-skylib/archive/f83cb8dd6f5658bc574ccd873e25197055265d1c.tar.gz"],
     },
 )
 
@@ -195,6 +196,7 @@ http_file(
 
 http_archive(
     name = "bazel_skylib",
+    # Commit f83cb8dd6f5658bc574ccd873e25197055265d1c of 2018-11-26
     sha256 = "ba5d15ca230efca96320085d8e4d58da826d1f81b444ef8afccd8b23e0799b52",
     strip_prefix = "bazel-skylib-f83cb8dd6f5658bc574ccd873e25197055265d1c",
     urls = [