Stop using datetime.utcnow() to fix warning

[yuvipanda]

/Users/yuvipanda/code/repo2docker/./repo2docker/buildpacks/conda/freeze.py:101: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).