Changes:
- Fixed `nix/status-go/desktop` builds
- Dropped nimbus wrapper for `status-go` for now
- Split `status-go` builds into subfolders: `mobile`, `desktop`
- Fixed shells for desktop builds: `linux`,`macos`,`windows`
- Added `make status-go-*` targets for building them
- Moved source management to `nix/status-go/source.nix`
- Moved `nix/status-go/build.nix` into `nix/status-go/mobile`
- Moved `nix/desktop/cmake/qtkeychain` to `nix/pkgs/qtkeychain-src`
- Moved `nix/desktop/linux/linuxdeployqt` to `nix/pkgs`
- Moved `nix/desktop/linux/appimagekit` to `nix/pkgs`
- Dropped `nix/tools/mkShell.nix` since it did almost nothing
- Dropped `nix/desktop/cmake/snorenotify` since it's broken
- Moved setup from `nix/tools/mkShell.nix` to `nix/shells.nix`
- Simplified `nix/mobile/ios/status-go-shell.nix`
- Simplified `nix/status-go/default.nix`
- Updated the `nix/DETAILS.md` and `nix/README.md`
- Moved known issues to `nix/KNOWN_ISSUES.md`
- Improved output of `nix/scripts/build.sh`
Signed-off-by: Jakub Sokołowski <jakub@status.im>
