nixos-install cannot find identities

[RSWilli]

I'm new to nixos and I'm installing a second system from a flake.

I'm using the nixos ISO file to install a new system. I have identities on a separate LUKS encrypted usb drive plugged into the system. I configured agenix in the following way:

age.identityPaths = [
    "/etc/ssh/ssh_host_ed25519_key"
    "/etc/ssh/ssh_host_rsa_key"
    "/run/media/nixos/installer-keys/install_key" # only on live ISO
];

The secrets are also keyed against the "install_key.pub".

and I run sudo nixos-install --flake .#main to install the system. After all the nix build logs, agenix logs:

[agenix] WARNING: config.age.identityPaths entry /run/media/nixos/installer-keys/install_key not present!

Does the nixos-install command create some kind of chroot so it doesn't have access to the usb drive? Or am I doing something else wrong?