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.

app.yml

@@ -5,6 +5,7 @@ settings:
   BASE_BUNDLE_IDENTIFIER: io.element.elementx
   ASSETCATALOG_COMPILER_GLOBAL_ACCENT_COLOR_NAME: "colors/accent-color"
   DEVELOPMENT_TEAM: 7J4U792NQT
+  IS_NIGHTLY_BUILD: false
 
   # Optional configuration to load accounts from Element Classic.
   CLASSIC_APP_GROUP_IDENTIFIER: group.im.vector