flake/dev: pin flake-compat to PR fixing shallow repo

Pin flake-compat to NixOS/flake-compat#75
which fixes NixOS/flake-compat#74

If the flake is a shallow git repo, such as when fetched in CI,
flake-compat was failing to run in nix-instantiate.

Flake lock file updates:

• Updated input 'flake-compat':
    'https://api.flakehub.com/f/pinned/edolstra/flake-compat/1.1.0/01948eb7-9cba-704f-bbf3-3fa956735b52/source.tar.gz?narHash=sha256-NeCCThCEP3eCl2l/%2B27kNNK7QrwZB1IJCrXfrbv5oqU%3D' (2024-12-04)
  → 'github:edolstra/flake-compat/b70558fcb45e248abd2a0add41cfb6464c94eb03?narHash=sha256-JtdaTv%2B1VIiZpLsULCSQte6Qg3ew5axbdpOYjo%2B8D2U%3D' (2025-07-12)

Author: MattSturgeon