run-tests: Allow to override python3 command to use via environment var.

tests/run-tests

@@ -9,7 +9,7 @@ if os.name == 'nt':
     CPYTHON3 = 'python3.exe'
     MP_PY = '../windows/micropython.exe'
 else:
-    CPYTHON3 = 'python3'
+    CPYTHON3 = os.getenv('MICROPY_CPYTHON3', 'python3')
     MP_PY = '../unix/micropython'
 
 def rm_f(fname):