interpreter: Say 'build' machine when adding languages for is_build_only

Even if it's the host machine from the perspective of the subproject,
it looks confusing when a dependency lookup a few lines down refers to
the build machine.

mesonbuild/interpreter/interpreter.py

@@ -1536,7 +1536,7 @@ def add_languages_for(self, args: T.List[str], required: bool, for_machine: Mach
         for lang in sorted(args, key=compilers.sort_clink):
             if lang in self.compilers[for_machine]:
                 continue
-            machine_name = for_machine.get_lower_case_name()
+            machine_name = 'build' if self.coredata.is_build_only else for_machine.get_lower_case_name()
             comp = self.coredata.compilers[for_machine].get(lang)
             if not comp:
                 try: