Changes:
- Adds a new `nix-gc` Makefile target for removing old packages
- Moves all `nix/*.sh` files to `nix/scripts/*.sh` to make things more tidy
- Renames `TARGET_OS` into `TARGET` and makes it effective only with `nix/scripts/shell.sh`
- Renames `target-os` Nix argument to just `target` and makes it effective only with `shell.nix`
- Drops `IN_CI_ENVIRONMENT` env variable which was useless
- Drops use of `target-os` argument outside of `shell.nix` (with few exceptions, but just in naming)
- `nix/platform.nix` has been made obsolete and removed
- Moves the definition of all major targets to `nix/targets.nix`
- Moves the definition of all major shells to `nix/shells.nix`
- Makes `default.nix` and `shell.nix` just thin wrappers around `nix/default.nix`
- `nix/nixpkgs-bootstrap.nix` has been moved to `nix/pkgs.nix`
- All package and tool overrides have been moved to `nix/pkgs.nix`
- Explicit passing of contents of `pkgs` has been removed in favor of `callPackage` doing it for us
- `nix/bootstrapped-shell.nix` has been moved to `nix/tools/mkShell.nix`
- A new `mergeSh` tool has been added to `pkgs` from `nix/tools/mergeSh.nix`
- This tool is used to merge shells created using `mkShell`
- `mobile/targets/jsbundle.nix` has been moved to `mobile/android/jsbundle/default.nix`
- Moves `status-go` version sanitization to `nix/status-go/utils.nix`
- Renames version to rawVersion and versionName to cleanVersion in status-go derivation
- Ports nix/mobile/ios/install-pods-and-status-go.sh to Nix sub-shells
- Moves adjustment of `inotify/max_user_watches` out into `scripts/inotify_fix.sh`
- Makes iOS builds use the Nix version of Fastlane
Signed-off-by: Jakub Sokołowski <jakub@status.im>
