[bazel] Use a specific version of Buildifier to make the build more reproducible.

Bug: skia:11111
Change-Id: Iea40c0bb9d3bdff31e48f0c0e7b164244c126ada
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/371760
Reviewed-by: Leandro Lovisolo <lovisolo@google.com>
diff --git a/WORKSPACE b/WORKSPACE
index 5659136..f8d5cbc 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -27,9 +27,9 @@
 
 http_archive(
     name = "com_github_bazelbuild_buildtools",
-    sha256 = "1dbb1f39c17b1cbc011cc22394e6e88b0de13ad101eb40047c603297286c8398",
-    strip_prefix = "buildtools-master",
-    url = "https://github.com/bazelbuild/buildtools/archive/master.zip",
+    sha256 = "2adaafee16c53b80adff742b88bc90b2a5e99bf6889a5d82f22ef66655dc467b",
+    strip_prefix = "buildtools-4.0.0",
+    url = "https://github.com/bazelbuild/buildtools/archive/4.0.0.zip",
 )
 
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