chore: add weekly flake.lock update workflow

[yvan-sraka]

Adds GitHub Action to automatically update flake.lock every Sunday. Uses DeterminateSystems/update-flake-lock to create PRs with updated dependencies, preventing large, painful updates like the recent 2-year gap... https://github.com/supabase/postgres/pull/1714

Targets develop branch with automated and dependencies labels. Can also be triggered manually via workflow_dispatch.