Set security.exec.allow to ['^asciidoctor$']

The default security policy doesn't allow executing asciidoctor.

config.toml

@@ -9,3 +9,7 @@ enableGitInfo = true
   BookComments = false
   BookRepo = 'https://gitlab.archlinux.org/archlinux/service-agreements'
   BookCommitPath = 'commit'
+
+[security]
+  [security.exec]
+    allow = ['^asciidoctor$']