Update go_version to 1.12.5

Upstream change is kubernetes/kubernetes#77528

WORKSPACE

@@ -21,7 +21,7 @@ load("@io_bazel_rules_go//go:deps.bzl", "go_rules_dependencies", "go_register_to
 go_rules_dependencies()
 
 go_register_toolchains(
-    go_version = "1.12.1",
+    go_version = "1.12.5",
 )
 
 load("@bazel_gazelle//:deps.bzl", "gazelle_dependencies")