buildman: Handle CROSS_COMPILE being in the environment

If CROSS_COMPILE is set in the environment passed to buildman,
testMakeEnvironment() fails. Fix this by removing the variable before
starting the test.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/buildman/test.py

@@ -650,6 +650,7 @@ class TestBuild(unittest.TestCase):
 
     def testMakeEnvironment(self):
         """Test the MakeEnvironment function"""
+        os.environ.pop('CROSS_COMPILE', None)
         tc = self.toolchains.Select('arm')
         env = tc.MakeEnvironment(False)
         self.assertEqual(env[b'CROSS_COMPILE'], b'arm-linux-')