Update procedures

sources.json

# run from parent directory
nix-shell -p niv --run "niv update"
# or
nix-shell -p niv --run "niv update nixpkgs -b master"
# or (check current release here https://nixos.org/download.html)
nix-shell -p niv --run "niv update nixpkgs -b release-..."

asterix-libs

nix-prefetch-git https://github.com/zoranbosnjak/asterix-libs.git \
    > nix/asterix-libs.json
# or
hsh=...
nix-prefetch-git --rev $hsh https://github.com/zoranbosnjak/asterix-libs.git \
    > nix/asterix-libs.json