use flake should prompt user for re-approval if flake.nix changed

[yshui]

Is your feature request related to a problem? Please describe.
Right now, direnv will only prompt for direnv allow if .envrc changed. If .envrc is use flake, user won't be prompted again if flake.nix changed. This means the user has to fully trust the upstream's flake.nix, resulting in a different threat model compared to using a normal .envrc.

Describe the solution you'd like
If .envrc is use flake, the user should be prompted again for direnv allow if flake.nix changed.

Describe alternatives you've considered
N/A

Additional context
N/A