Allow usage with nix daemon

[AleksanderGondek]

This commit removes the restriction of running the project only via single-user nix installation.
To ensure disruptor is run with appropriate settings, a comparison is made betweeen global nix configuration and those specified in scripts/nix.conf file - any differences is clearly communicated to user

[AleksanderGondek]

Example of warning:

$ ./nix-shell.sh
The configuration of host nix daemon differs from the expected one.
These are the offending settings:
{ keep-outputs = "'true' change to => 'false';"; require-sigs = "'false' change to => 'true';"; }
---
Please update your configuration otherwise the shell may not work as intended.
direnv: loading ~/projects/disruptor/.envrc

[nix-shell:~/projects/disruptor]$

[r2r-dev]

[r2r-dev]

Why not add --experimental-features nix-command to the command above?

Also this condition can be reached in cases when installed nix version is below 2.4.

[AleksanderGondek]

I will add the flag, I have been stuck thinking about unprivledged user but in other cases it saves the user a trip to nix.conf.

As to nix version... isn't it available even in version 2.0?
https://github.com/NixOS/nix/blob/2.0/src/nix/show-config.cc

[r2r-dev]

I like where this is heading. I'm ok with merging these changes. However, I think large chunks of this solution could be moved to shell.nix entirely. Some references:
https://github.com/NixOS/nixpkgs/blob/420a0cce9358acf8b9c9c2ae3a0e421097f84c50/nixos/modules/services/misc/nix-daemon.nix#L72
https://github.com/cheronobyl/nixflk/blob/27513eeed119ccb82eee0abb60b93f0cd9758473/shell.nix#L25

In general, as soon as we know that nix binary is available in any shape or form, we can leverage initial nix expression, e.g. shell.nix to perform the rest of checks, and config set-up. You can even import nix library to ease the pain:
https://github.com/nix-community/nixpkgs.lib