Placed GitHub release first for @skylib.

See bazelbuild/bazel-skylib#164.
diff --git a/cc/repositories.bzl b/cc/repositories.bzl
index d892188..ebc972a 100644
--- a/cc/repositories.bzl
+++ b/cc/repositories.bzl
@@ -9,8 +9,8 @@
         sha256 = "2ef429f5d7ce7111263289644d233707dba35e39696377ebab8b0bc701f7818e",
         strip_prefix = "bazel-skylib-0.8.0",
         urls = [
-            "https://mirror.bazel.build/github.com/bazelbuild/bazel-skylib/archive/0.8.0.tar.gz",
             "https://github.com/bazelbuild/bazel-skylib/archive/0.8.0.tar.gz",
+            "https://mirror.bazel.build/github.com/bazelbuild/bazel-skylib/archive/0.8.0.tar.gz",
         ],
     )