base: derive _CUSTOMER from the builtin host label

The `cmk/customer` builtin host label is contributed by the managed-services
edition alone, from the same `conf.d/customer.mk` that base reads into
`current_customer`. Its presence is therefore an exact stand-in for the edition
check, and since builtin labels are merged last in `labels_of_host()` a user
label cannot shadow it.

This removes one of two reads of `ConfigCache.edition`.

Change-Id: I4c67a719dc6e0fa45e24f4da2ec516126cbff123

Author: mo-ki

cmk/base/config/_impl.py

@@ -110,7 +110,7 @@
 from cmk.piggyback import backend as piggyback_backend
 from cmk.ruleset_matcher import matcher as ruleset_matcher
 from cmk.ruleset_matcher import tuple_rulesets
-from cmk.ruleset_matcher.labels import LabelManager, Labels, LabelSources
+from cmk.ruleset_matcher.labels import BuiltinLabelsKey, LabelManager, Labels, LabelSources
 from cmk.ruleset_matcher.matcher import (
     RulesetMatcher,
     RulesetName,
@@ -2715,8 +2715,8 @@ def get_host_attributes(
         if actions := self.icons_and_actions(hostname):
             attrs["_ACTIONS"] = ",".join(actions)
 
-        if self.edition is cmk_version.Edition.ULTIMATEMT:
-            attrs["_CUSTOMER"] = self._loaded_config.current_customer
+        if customer := self.label_manager.labels_of_host(hostname).get(BuiltinLabelsKey.CUSTOMER):
+            attrs["_CUSTOMER"] = customer
 
         return attrs