test_axon.py: add NEST_ASYNCIO:0 to env to fix issue

Bittensor uses nest_asyncio by default. This doesn't work well with
uvicorn, that is used in the implementation of axon.

By disabling nest_asyncio the miner now runs without errors.

tests/e2e_tests/multistep/test_axon.py

@@ -95,6 +95,7 @@ async def test_axon(local_chain):
         *args,
         stdout=asyncio.subprocess.PIPE,
         stderr=asyncio.subprocess.PIPE,
+        env={"NEST_ASYNCIO": "0"},
     )
     await asyncio.sleep(1)  # wait for the miner to start up or fail
     if axon_process.returncode != None: