`make delete_environment` on windows throws an error message

```
>>> make delete_environment
del environment.Windows-10-10.0.19042-SP0.lock.yml
process_begin: CreateProcess(NULL, del environment.Windows-10-10.0.19042-SP0.lock.yml, ...) failed.
make (e=2): The system cannot find the file specified.
make: *** [Makefile.envs:34: delete_environment] Error 2
```

the `del` should ignore a missing file here