Bug 1412356 - Invoke `mach build` for multi locale builds; r=chmanchester

This should be functionally identical to `make -f client.mk`.

MozReview-Commit-ID: GfKHtezLBNL

--HG--
extra : rebase_source : e7965718b16d22055abdab2838e97cae63330ce9

testing/mozharness/mozharness/mozilla/l10n/multi_locale_build.py

@@ -127,9 +127,10 @@ class MultiLocaleBuild(LocalesMixin, MercurialScript):
         self.copyfile(os.path.join(dirs['abs_work_dir'], c['mozconfig']),
                       os.path.join(dirs['abs_mozilla_dir'], 'mozconfig'),
                       error_level=FATAL)
-        command = "make -f client.mk build"
+
+        mach = os.path.join(dirs['abs_mozilla_dir'], 'mach')
         env = self.query_env()
-        if self._process_command(command=command,
+        if self._process_command(command=[sys.executable, mach, 'build'],
                                  cwd=dirs['abs_mozilla_dir'],
                                  env=env, error_list=MakefileErrorList):
             self.fatal("Erroring out after the build failed.")