Bump buildifier for bazelci to 5.1.0 (#276)

* Bump buildifier for bazelci

* Use cfg='exec'

* manually fix cfg=exec in vendored files

I don't like doing this (manually editing vendored files) but don't know how to exclude these files from the buildifier bazelci task.

Author: pcj

.bazelci/presubmit.yml

@@ -1,6 +1,6 @@
 ---
 buildifier:
-  version: 4.2.3
+  version: 5.1.0
   warnings: "-function-docstring,-function-docstring-header,-module-docstring,-function-docstring-args,-list-append"
 bazel: 5.1.1
 tasks:

cmd/gencopy/gencopy.bzl

@@ -28,7 +28,7 @@ gencopy_attrs = {
         doc = "The gencopy binary",
         default = str(Label("//cmd/gencopy")),
         allow_single_file = True,
-        cfg = "host",
+        cfg = "exec",
         executable = True,
     ),
 }

rules/example.bzl

@@ -59,7 +59,7 @@ _examplegen = rule(
             default = "//cmd/examplegen",
             allow_single_file = True,
             executable = True,
-            cfg = "host",
+            cfg = "exec",
         ),
     },
     outputs = {

rules/proto_gazelle.bzl

@@ -79,7 +79,7 @@ _gazelle_runner = rule(
         "gazelle": attr.label(
             default = "@bazel_gazelle//cmd/gazelle",
             executable = True,
-            cfg = "host",
+            cfg = "exec",
         ),
         "command": attr.string(
             values = [