When lockFileMaintenance and the nix manager are enabled, flake.lock files
are only refreshed if flake.nix contains a string like
"github:NixOS/nixpkgs/anything" (including the double quotes.
anything can be replaced with any alpha-numeric string of characters).
I would expect that when lockFileMaintenance and the nix manager are
enabled, then flake.lock will always be refreshed, regardless of the
contents/inputs defined in flake.nix.
A workaround for this bug is to inject a dummy string to satisfy Renovate's hard-coded regex:
echo '# Trick renovate into working: "github:NixOS/nixpkgs/nixpkgs-unstable"' >> flake.nix