@fasterthanlime@msfjarvis apologize for being slightly pedantic here: when using flakes, nix requires downloading all flake inputs transitively before it can evaluate derivations.
Derivation inputs are only downloaded when trying to build the derivation.
Unless IFD (input from derivation) is triggered, you technically don’t need toolchains present just to evaluate things (eg via nix build .# --dry-run).
Happy to take a look if crane seems to be at fault triggering IFD