docs: Update install methods in README.md

[popey]

The 'community maintained' link for Nix has moved to https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/sy/syft/package.nix (similarly for grype)

The snap is established now, and while 'proof of concept' is not packaged by "us" (but "me"), it might be worth mentioning it. Both Syft and Grype snaps have users cross ~30 countries. 100% of them are on the latest release. The Snap store even has a natty little badge:

deb-get is a popular way to install debs of stable software from upstream developers where the packages are not in the traditional apt archive. We should mention it, because it allows users to get updates without hunting down the deb manually. It has a deb-get upgrade option.

$ deb-get show syft
Syft
  Package:      syft
  Repository:   01-main
  Updater:      deb-get
  Installed:    1.11.1
  Published:    1.11.1
  Architecture: amd64 arm64
  Download:     https://github.com/anchore/syft/releases/download/v1.11.1/syft_1.11.1_linux_amd64.deb
  Website:      https://github.com/anchore/syft
  Summary:      CLI tool and library for generating a Software Bill of Materials from container images and filesystems.