Bug 1721755 - Only set EARLY_BETA_OR_EARLIER on beta. r=firefox-build-system-reviewers,andi

This matches the spirit of what BUILDING_RELEASE used to do before bug
1597834.

Differential Revision: https://phabricator.services.mozilla.com/D120563

build/moz.configure/init.configure

@@ -1244,7 +1244,7 @@ def milestone(build_env, build_project, version_path, _):
                 die(
                     "Only the EARLY_BETA_OR_EARLIER variable can be set in build/defines.sh"
                 )
-            if value:
+            if value and "b" in app_version_display:
                 is_early_beta_or_earlier = True
 
     # Only expose the major version milestone in the UA string and hide the