asciidoc.bzl: Migrate cfg = "host" to "exec"

No-op cleanup: cfg = "host" is a deprecated alias for cfg = "exec"

Change-Id: I592abf8a1f393d40e52cfb383552219334387580
Release-Notes: skip
diff --git a/tools/bzl/asciidoc.bzl b/tools/bzl/asciidoc.bzl
index 7977cf0..0858d60 100644
--- a/tools/bzl/asciidoc.bzl
+++ b/tools/bzl/asciidoc.bzl
@@ -122,7 +122,7 @@
     ),
     "_exe": attr.label(
         default = Label("//java/com/google/gerrit/asciidoctor:asciidoc"),
-        cfg = "host",
+        cfg = "exec",
         allow_files = True,
         executable = True,
     ),