Fix incompatibility with --incompatible_disable_starlark_host_transitions (#1275)

See bazelbuild/bazel#17032

Caught by
bazelbuild/bazel-central-registry#4402 (comment)

Author: meteorcloudy

bazel/protobuf.bzl

@@ -110,13 +110,13 @@ cc_proto_gen_validate = rule(
             default = [Label("@com_googlesource_code_re2//:re2")],
         ),
         "_protoc": attr.label(
-            cfg = "host",
+            cfg = "exec",
             default = Label("@com_google_protobuf//:protoc"),
             executable = True,
             allow_single_file = True,
         ),
         "_plugin": attr.label(
-            cfg = "host",
+            cfg = "exec",
             default = Label("@com_envoyproxy_protoc_gen_validate//:protoc-gen-validate"),
             allow_files = True,
             executable = True,
@@ -171,13 +171,13 @@ _java_proto_gen_validate_aspect = aspect(
     attr_aspects = ["deps"],
     attrs = {
         "_protoc": attr.label(
-            cfg = "host",
+            cfg = "exec",
             default = Label("@com_google_protobuf//:protoc"),
             executable = True,
             allow_single_file = True,
         ),
         "_plugin": attr.label(
-            cfg = "host",
+            cfg = "exec",
             default = Label("@com_envoyproxy_protoc_gen_validate//:protoc-gen-validate"),
             allow_files = True,
             executable = True,