Introduce a build setting to detect Nightly builds. (#5852)

* Allow the AppIconBanner tool to be called with purely the build number.

And use that for the Element Pro Nightly.

* Use a build setting to determine Nightly builds.

ShareExtension/SupportingFiles/Info.plist

@@ -51,6 +51,8 @@
 	<string>$(APP_GROUP_IDENTIFIER)</string>
 	<key>baseBundleIdentifier</key>
 	<string>$(BASE_BUNDLE_IDENTIFIER)</string>
+	<key>isNightlyBuild</key>
+	<string>$(IS_NIGHTLY_BUILD)</string>
 	<key>keychainAccessGroupIdentifier</key>
 	<string>$(KEYCHAIN_ACCESS_GROUP_IDENTIFIER)</string>
 	<key>productionAppName</key>

ShareExtension/SupportingFiles/target.yml

@@ -49,6 +49,7 @@ targets:
         baseBundleIdentifier: $(BASE_BUNDLE_IDENTIFIER)
         keychainAccessGroupIdentifier: $(KEYCHAIN_ACCESS_GROUP_IDENTIFIER)
         productionAppName: $(PRODUCTION_APP_NAME)
+        isNightlyBuild: $(IS_NIGHTLY_BUILD)
         NSExtension:
           NSExtensionPointIdentifier: com.apple.share-services
           NSExtensionPrincipalClass: $(PRODUCT_MODULE_NAME).ShareExtensionViewController