python::requirements Error: Parameter cwd failed on Exec for virtualenv=system

while using
```ruby
python::requirements { "${home_dir}/pip.requirements.txt":
        virtualenv => 'system',
        require => File["${home_dir}/pip.requirements.txt"]
}
```

I get the following errors
```shell
Error: Parameter cwd failed on Exec[python_requirements_update_/home/salimane/pip.requirements.txt]: cwd must be a fully qualified path
```
```shell
command not found `which
```

This pr fix those errors
Thanks