Add flake support to make this easy to consume for nixos and nix enabled devices

[f0rdprefect]

I needed this to run on my NixOS machine. So I vibed this flake which works fine for me.

Use nix develop to get a development shell which can then be used to do the backup and restore

[mrrfv]

Looks good at first glance, thank you. I'll try to test the flake next weekend.

I see a few minor issues, though:

• The shellHook sets a variable called OAB_COMPRESSION_LEVEL, which isn't used anywhere in the project. I presume this was a hallucination. TMPDIR may not be required either.
• It's probably unnecessary to check if adb is available in the environment.
• I'd remove the "Environment variables for automation" part and refer to the README instead, to avoid redundancy.

Beyond that, I think this will be a nice enhancement!