АБ
repl () {
source="$(nix flake prefetch --json "$1" | /nix/store/3ihyylq3k76xrw6mki0dsgmi4rrxmxkr-jq-1.6-bin/bin/jq -r .storePath)"
TEMP="$(mktemp --suffix=.nix)"
echo "let self = builtins.getFlake \"$source\"; in self // self.legacyPackages.\${builtins.currentSystem} or { } // self.packages.\${builtins.currentSystem} or { }" > "$TEMP"
nix repl "$TEMP"
rm "$TEMP"
}
И потом просто
repl github:nixos/nixpkgs/540dccb2aeaffa9dc69bfdc41c55abd7ccc6baa3