Pin mailman to ai3 repo

Buster's mailman version is greater than ai3's now. Force mailman to
come from ai3 instead.

build.sh

@@ -50,6 +50,13 @@ mailman mailman/default_server_language select en
 mailman mailman/site_languages multiselect ar (Arabic), ca (Catalan), cs (Czech), da (Danish), de (German), en (English), es (Spanish), et (Estonian), eu (Basque), fi (Finnish), fr (French), hr (Croatian), hu (Hungarian), ia (Interlingua), it (Italian), ja (Japanese), ko (Korean), lt (Lithuanian), nl (Dutch), no (Norwegian), pl (Polish), pt (Portuguese), pt_BR (Brasilian Portuguese), ro (Romanian), ru (Russian), sl (Slovenian), sr (Serbian), sv (Swedish), tr (Turkish), uk (Ukrainian), vi (Vietnamese), zh_CN (Chinese - China), zh_TW (Chinese - Taiwan)
 EOF
 
+# Always prefer mailman from ai3
+cat > /etc/apt/preferences.d/90mailman <<EOF
+Package: mailman
+Pin: release n=ai3
+Pin-Priority: 1001
+EOF
+
 install_packages ${BUILD_PACKAGES} ${PACKAGES}
 
 # Install the configuration, overlayed over /etc.