invenio-cli init creates empty pipfile on root directory (outside of instance)

Package version (if known): 0.23.0

## Describe the bug
When creating a new instance with `invenio-cli init rdm` a Pipfile is created in the root directory, e.g.:

```
$ ls
Pipfile instance_name/
```

And the file contains nothing:

```
cat Pipfile
[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[packages]

[dev-packages]

[requires]
python_version = "3.7"
```

The useful one is inside the instance folder...

## Steps to Reproduce
1. Install invenio-cli
2. Bootstrap an instance
3. Check the source folder after `invenio-cli init`

## Expected behavior

No useless Pipfile is created