Fix executing script name on Windows

[cswartzvi]

Hello, first time contributor. This is a lovely project. On Windows, and specifically when installed via conda, doit --help displays the path of the entire executing script (see below). This PR corrects that by using os.path.split vice .split('/'). Question: why not just display the name as doit? It looks like there was considerable thought behind persevering the name of the executing script, I was curious as to its use case.

❯ doit --help 
doit -- automation tool
http://pydoit.org

Commands
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py clean             clean action / remove targets
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py dumpdb            dump dependency DB
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py forget            clear successful run status from internal DB
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py help              show help
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py ignore            ignore task (skip) on subsequent runs
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py info              show info about a task
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py list              list tasks from dodo file
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py reset-dep         recompute and save the state of file dependencies without executing actions
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py run               run tasks
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py strace            use strace to list file_deps and targets
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py tabcompletion     generate script for tab-completion

  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py help              show help / reference
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py help task         show help on task dictionary fields
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py help <command>    show command usage
  C:\Users\Charles\AppData\Local\mambaforge\Scripts\doit-script.py help <task-name>  show task usage