Change default directory for `Flow.runFlow`

[sir4ur0n]

Currently Flow.runFlow uses /tmp/funflow/store/ as default directory.

As discussed over Slack this has a couple of limitations:

• this is out of the working directory, which may be a surprising side effect
• this can lead to conflicts / weird behaviors if 2 concurrent or parallel Funflow runs interact with it

I suggest we change it to the current working directory, e.g. $(pwd)/tmp/funflow/store/.

@dorranh @GuillaumeDesforges