Changes:
- Add `nix/DETAILS.md` for more in-depth info
- Rename some of `config.status-im` variables
- Drop `env` argument for Android build
- Use `overlays` instead of `packageOverrides`
- Move the `pkgs` overlay to `nix/overlay.nix`
- Move `nix/status-go/utils.nix` to `nix/tools`
- Make `shell.nix` use the `shells.default` only
- Use `default.nix` as target for `nix/scripts/shell.sh`
- Make `nix/scripts/shell.sh` use `--attr` instead of `target`
- Drop the `target` argument in favour of using `--attr`
- Drop unnecessary `src` from `nix/mobile/ios/default.nix`
- Move `mkShell` and `mergeSh` under `lib`
- Move `patched-go` package to `nix/pkgs` directory
- Move `gomobile` package to `nix/pkgs` directory
- Move `ANDROID_ABI_SPLIT` to `config.status-im.android.abi-split`
- Move `ANDROID_ABI_INCLUDE to `config.status-im.android.abi-include`
Signed-off-by: Jakub Sokołowski <jakub@status.im>
