This should also fix issues with MacOS sandboxing:
```
sandbox-exec: execvp() of '/nix/store/...-bash-4.4-p23/bin/bash' failed: Operation not permitted
```
https://github.com/cachix/install-nix-action/issues/15
Signed-off-by: Jakub Sokołowski <jakub@status.im>
Changes:
- Add missing lazy assignment for OS_NAME, fixes double Nix shells
- Moved `scripts/add-nix-gcroots.sh` to `nix/scripts/gcroots.sh`
- Moved Nix package manager setup to `nix/scripts/setup.sh`
- Created `nix/scripts/source.sh` to reuse in all Nix scripts
- Created `STARTING_GUIDE.md` with instructions for contributors
- Created `scripts/colors.sh` for definition of shell colors
- Removed `scripts/setup` in favor of `nix/scripts/setup.sh`
- Removed all of `scripts/lib` since it was useless
Signed-off-by: Jakub Sokołowski <jakub@status.im>
Jakub Sokołowski