Adding a 'just symlink to cgi-bin' nix-serve.cgi

Currently nix-serve as it is supposed to be run as a server. Add an option to use it as a CGI script with any HTTP server. The CGI can be just symlinked/copied to cgi-bin with zero configuration.
OK, I agree, it's an overkill.

So it will be bzip2 and nix.

Otherwise, if you use this as CGI, Apache runs in a poor environment without bzip2 and nix-store in PATH. Obviously, creation of bzip2-compressed NAR fails…