wrap with FHS for subcommands that run embedded executable

[hallettj]

The package in this repo provides a precompiled, statically-linked binary. However although the main binary is statically-linked, it includes an embedded binary that is required for some subcommands that is dynamically linked. This means that those subcommands don't work on NixOS if the user is not using nix-ld.

The embedded binary does not have any runtime dependencies - it only needs its interpreter, ld. Ideally we would patch the binary to replace references of the form /lib64/ld-linux-x86-64.so.2 with corresponding nix store paths. But I don't know how to do that. So instead this change wraps the binary with an FHS which makes the interpreter available at the expected path in a fake filesystem.

I also refactored flake.nix to remove the use of flake-utils just because I prefer it this way.

Fixes hasura/graphql-engine#10725