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.

Variants/Nightly/nightly.yml

@@ -1,6 +1,7 @@
 settings:
   APP_GROUP_IDENTIFIER: group.io.element.nightly
   BASE_BUNDLE_IDENTIFIER: io.element.elementx.nightly
+  IS_NIGHTLY_BUILD: true
 
 targets:
   ElementX:
@@ -8,4 +9,4 @@ targets:
       base:
         ASSETCATALOG_COMPILER_APPICON_NAME: NightlyAppIcon
     sources:
-    - path: Resources
+    - path: Resources