iosevka: get `set` value from `privateBuildPlan`, if it's an attrset (#401520)

  #   sequence = "+>"

  # '';

  extraParameters ? null,

  # Custom font set name. Required if any custom settings above.

  set ? null,

  # Custom font set name. Required if `extraParameters` is used and/or

  # if `privateBuildPlan` is a TOML string.

  set ? privateBuildPlan.family or null,

}:

assert (builtins.isAttrs privateBuildPlan) -> builtins.hasAttr "family" privateBuildPlan;

assert (privateBuildPlan != null) -> set != null;

assert (extraParameters != null) -> set != null;