Update to rules_go and buildtools (grpc-ecosystem#956)

* Update to rules_go v0.18.6

This fixes the build with Bazel 0.27.

* Update bazelbuild/buildtools to v0.26.

This is also required for Bazel 0.27, it seems.

Fixes grpc-ecosystem#955

BUILD

@@ -12,6 +12,7 @@ buildifier(
 
 # gazelle:exclude third_party
 # gazelle:exclude vendor
+# gazelle:exclude _output
 # gazelle:prefix github.com/grpc-ecosystem/grpc-gateway
 
 gazelle(name = "gazelle")

WORKSPACE

@@ -4,8 +4,11 @@ load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
 http_archive(
     name = "io_bazel_rules_go",
-    sha256 = "6776d68ebb897625dead17ae510eac3d5f6342367327875210df44dbe2aeeb19",
-    urls = ["https://github.com/bazelbuild/rules_go/releases/download/0.17.1/rules_go-0.17.1.tar.gz"],
+    sha256 = "f04d2373bcaf8aa09bccb08a98a57e721306c8f6043a2a0ee610fd6853dcde3d",
+    urls = [
+        "https://storage.googleapis.com/bazel-mirror/github.com/bazelbuild/rules_go/releases/download/0.18.6/rules_go-0.18.6.tar.gz",
+        "https://github.com/bazelbuild/rules_go/releases/download/0.18.6/rules_go-0.18.6.tar.gz",
+    ],
 )
 
 http_archive(
@@ -61,8 +64,9 @@ go_repository(
 
 http_archive(
     name = "com_github_bazelbuild_buildtools",
-    strip_prefix = "buildtools-bf564b4925ab5876a3f64d8b90fab7f769013d42",
-    url = "https://github.com/bazelbuild/buildtools/archive/bf564b4925ab5876a3f64d8b90fab7f769013d42.zip",
+    sha256 = "86592d703ecbe0c5cbb5139333a63268cf58d7efd2c459c8be8e69e77d135e29",
+    strip_prefix = "buildtools-0.26.0",
+    urls = ["https://github.com/bazelbuild/buildtools/archive/0.26.0.tar.gz"],
 )
 
 load("@com_github_bazelbuild_buildtools//buildifier:deps.bzl", "buildifier_dependencies")