Run:
nix develop
If you have direnv installed, just enter the directory and it should automatically load it for you.
Now you can run SBCL with alexandria on the path:
sbcl --script <<EOF
(require "asdf")
(asdf:load-system "alexandria")
(print (alexandria:iota 3))
EOF
You could also for example run emacs from this shell to have access to SBCL from within it.
If you see either of these two errors:
error: experimental Nix feature 'nix-command' is disabled; use '--extra-experimental-features nix-command' to override
error: experimental Nix feature 'flakes' is disabled; use '--extra-experimental-features flakes' to override
Try adding this to any ‘nix’ command:
nix --extra-experimental-features 'flakes nix-command' ...