AFAICT it is free, though whoever maintains that package forgot to include a copy of the license text.
rust-once-cell-1 can't be built because it now depends on rust-parking-lot-core-0.9. This can't be built because it now requires rust-windows-sys-0.36 according to the crates page (https://crates.io/crates/parking_lot_core/0.9.3/dependencies). This package definition can be found in the guix channel that Must Not Be Named (right next to firefox), and is not present in the main guix repo.
I would guess the reasoning behind this is that rust-windows-sys requires some kind of nonfree code in its dependencies (perhaps rust-windows-aarch64-msvc-0.36?).
If I'm wrong about this and the package does meet the Guix repo standards, then the alacritty problem can be fixed by incorporating the rust-windows-sys-0.36 package definition into (gnu packages crates-io).
Another solution would be to remove the dependency rust-windows-sys from rust-parking-lot-core-0.9 by patching the Cargo.toml appropriately (this dependency is only required for compiling to Windows targets).
Greetings,
MAxime.