run_unittests: fix undefined variable in error message

In commit fe973d9fc4, some uses of p got
rewritten to compiler.language, but not all. We'd still raise an error
message, but for the wrong thing. o_O

run_unittests.py

@@ -6417,7 +6417,7 @@ class LinuxlikeTests(BasePlatformTests):
         elif compiler.language == 'cpp':
             env_flag_name = 'CXXFLAGS'
         else:
-            raise NotImplementedError('Language {} not defined.'.format(p))
+            raise NotImplementedError('Language {} not defined.'.format(compiler.language))
         env = {}
         env[env_flag_name] = cmd_std
         with self.assertRaises((subprocess.CalledProcessError, mesonbuild.mesonlib.EnvironmentException),