Update bazel-toolchains to 3.1.0

The version of bazel-toolchain in the current WORKSPACE file has some invalid escape sequences (https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/1756#951353b2-9c81-4819-b89e-e448b043f284). Bazel itself uses bazel-toolchains 3.1.0 right now, and those invalid escape sequences have been fixed some time before 3.1.0.

Change-Id: I013dfb1202bb2cbecd0d479e0fcd9e59a80ce929
(cherry picked from commit f100cda91ea0278bf2d4b1e68f18d35779d3209e)
diff --git a/WORKSPACE b/WORKSPACE
index 73e8874..2f8c883 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -8,11 +8,11 @@
 
 http_archive(
     name = "bazel_toolchains",
-    sha256 = "88e818f9f03628eef609c8429c210ecf265ffe46c2af095f36c7ef8b1855fef5",
-    strip_prefix = "bazel-toolchains-92dd8a7a518a2fb7ba992d47c8b38299fe0be825",
+    sha256 = "726b5423e1c7a3866a3a6d68e7123b4a955e9fcbe912a51e0f737e6dab1d0af2",
+    strip_prefix = "bazel-toolchains-3.1.0",
     urls = [
-        "https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/92dd8a7a518a2fb7ba992d47c8b38299fe0be825.tar.gz",
-        "https://github.com/bazelbuild/bazel-toolchains/archive/92dd8a7a518a2fb7ba992d47c8b38299fe0be825.tar.gz",
+        "https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/releases/download/3.1.0/bazel-toolchains-3.1.0.tar.gz",
+        "https://github.com/bazelbuild/bazel-toolchains/releases/download/3.1.0/bazel-toolchains-3.1.0.tar.gz",
     ],
 )