devbox update fail when git flake are present about devbox HOT 3 OPEN

There seem to be multiple problems here:

1. func (f *File) FetchResolvedPackage(pkg string) (*Package, error) { fails skip flakes
2. If devbox shell has not been run then devbox update will fail because the ./.devbox/gen/flake/flake.nix file is missing
3. Devbox issues nix build calls with --impure, but nix flake update is not specified with --impure. That said, I don't know if the semantics of nix flake update will be the same as devbox update performing nix build --refresh ... for flakerefs

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Might this be solved by:

1. adding a new method like IsRunX called IsFlake and skipping flakes in FetchResolvedPackage
2. pass installMode down from ensureStateIsUpToDate to installNixPackagesToStore and if installMode == update add --refresh to the command args

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@Lagoja FYI

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