update bazel-toolchains for Bazel 0.20.0

This is one step towards fixing remote execution CI failures.

RELNOTES: None
PiperOrigin-RevId: 224500232
diff --git a/WORKSPACE b/WORKSPACE
index f55bab9..fdd8495 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -168,11 +168,11 @@
 
 http_archive(
     name = "bazel_toolchains",
-    sha256 = "fa1459abc7d89db728da424176f5f424e78cb8ad7a3d03d8bfa0c5c4a56b7398",
-    strip_prefix = "bazel-toolchains-42619b5476b7c8a2f5117f127d5772cc46da2d1d",
+    sha256 = "07a81ee03f5feae354c9f98c884e8e886914856fb2b6a63cba4619ef10aaaf0b",
+    strip_prefix = "bazel-toolchains-31b5dc8c4e9c7fd3f5f4d04c6714f2ce87b126c1",
     urls = [
-        "https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/42619b5476b7c8a2f5117f127d5772cc46da2d1d.tar.gz",
-        "https://github.com/bazelbuild/bazel-toolchains/archive/42619b5476b7c8a2f5117f127d5772cc46da2d1d.tar.gz",
+        "https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/31b5dc8c4e9c7fd3f5f4d04c6714f2ce87b126c1.tar.gz",
+        "https://github.com/bazelbuild/bazel-toolchains/archive/31b5dc8c4e9c7fd3f5f4d04c6714f2ce87b126c1.tar.gz",
     ],
 )