No fallback to powershell / cmd?

Since 0.8.0, mprocs no longer starts the processes. Logs:

```
ERROR [portable_pty::win::psuedocon] CreateProcessW `"pwsh.exe -Command \"ls -lah\"\0"` in cwd `Some("R:\\\0")` failed: The system cannot find the file specified. (os error 2)
```

```yaml
procs:
  ls:
    shell: "ls -lah"
```

It would be great to fallback to `powershell` / `cmd` in case `pwsh` is missing.