Just ran into this myself. Longer term I think fwupd’s nixos module needs to adjust to be RFC42 compliant (rfcs/0042-config-option.md at 553b132ca05e0ad19b563b80b08d17330df205cf · NixOS/rfcs · GitHub) so these configs can be tweaked more easily. But in the meantime, what I did was put an override in my config that includes the default from the module source code, plus my change:
environment.etc."fwupd/uefi_capsule.conf" = lib.mkForce {
source = pkgs.writeText "uefi_capsule.conf" ''
[uefi_capsule]
OverrideESPMountPoint=${config.boot.loader.efi.efiSysMountPoint}
DisableCapsuleUpdateOnDisk=true
'';
};
This worked for me, I just updated my Framework’s firmware through a couple versions with no issues.