exec: es-start.bat: cannot run executable found relative to current directory

Isn't this a new restriction introduced in go 1.19?
https://tip.golang.org/doc/go1.19#os-exec-path
Looks like a bug like this one: https://github.com/syncthing/syncthing/issues/8499

I add to put a full path instead of a relative one. This should be documented?

do:
```
exec c:\test\es-start.bat {
startup
}
```

instead of:

```
exec es-start.bat {
startup
}
```