Update bazel-toolchains pin (#168)

This fixed Bazel CI failures (bazelbuild/bazel#6742) such as https://buildkite.com/bazel/bazel-with-downstream-projects-bazel/builds/613#820af988-4113-421f-b3f1-cef2f05d272c
diff --git a/WORKSPACE b/WORKSPACE
index 931b48c..027f079 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -31,11 +31,11 @@
 
 http_archive(
     name = "bazel_toolchains",
-    sha256 = "4ab012a06e80172b1d2cc68a69f12237ba2c4eb47ba34cb8099830d3b8c43dbc",
-    strip_prefix = "bazel-toolchains-646207624ed58c9dc658a135e40e578f8bbabf64",
+    sha256 = "7e85a14821536bc24e04610d309002056f278113c6cc82f1059a609361812431",
+    strip_prefix = "bazel-toolchains-bc0091adceaf4642192a8dcfc46e3ae3e4560ea7",
     urls = [
-        "https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/646207624ed58c9dc658a135e40e578f8bbabf64.tar.gz",
-        "https://github.com/bazelbuild/bazel-toolchains/archive/646207624ed58c9dc658a135e40e578f8bbabf64.tar.gz",
+        "https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/bc0091adceaf4642192a8dcfc46e3ae3e4560ea7.tar.gz",
+        "https://github.com/bazelbuild/bazel-toolchains/archive/bc0091adceaf4642192a8dcfc46e3ae3e4560ea7.tar.gz",
     ],
 )