Use default value cmd.exe for ENV["COMSPEC"]

[fonsp]

No description provided.

[fonsp]

This is two changes in one:

1. The default value for get(ENV, "COMSPEC", "cmd") was not used
2. Using cmd.exe instead of cmd means that it will work from WSL (which I will submit a 2nd PR for)

EDIT: 1. is also done by #10

[tpapp]

Thanks!