Fix nightly pipeline

[smelc]

Since its introduction in https://github.com/tweag/nixpkgs-graph-explorer/pull/102, the nightly pipeline does not work, as visible in the list of runs of actions (look for Nightly entries).

This issue keeps track of making it work. At the moment the output from the failing step doesn't give a hint:

# We write the output to a file, not to standard output,
  # We write the output to a file, not to standard output,
  # to keep log size small.
  nix develop --command bash -c 'cd core && poetry run python -m explorer.extract graph.json'
  wc -l graph.json
  shell: /usr/bin/bash -e {0}
trace: warning: qt5 now uses makeScopeWithSplicing which does not have "overrideScope'", use "overrideScope".
Error: Process completed with exit code 14[3](https://github.com/tweag/nixpkgs-graph-explorer/actions/runs/5041666607/jobs/9041557263#step:5:3).

[GuillaumeDesforges]

Thanks @smelc. Possibly, GitHub thinks we're mining bitcoin 😄

We might try to run the pipeline on a minimal flake instead of the whole github:nixos/nixpkgs/master flake. It can be done creating a minimal flake in the repo and using the path:path/to/flake flake reference.

[smelc]

We might try to run the pipeline on a minimal flake instead of the whole github:nixos/nixpkgs/master

@GuillaumeDesforges> yep indeed, the pipeline fails as follows:

which likely indicates that it simply runs out of memory (the GitHub runners being pretty cheap machines, that's not a surprise)

[GuillaumeDesforges]

https://github.com/tweag/nixpkgs-graph-explorer/pull/108 could help fix this issue as it consumes way less memory.