Channel addition test fails on aarch64/ppc64le

[QuLogic]

On aarch64 and ppc64le, trying to build farver 2.0.1, the channel addition test fails with:

  -- 1. Failure: adding channel works (@test-manip.R#30)  ------------------------
  add_to_channel(codes_alpha, "alpha", (1:10)/10) not equal to encode_colour(cols, alpha = alpha + (1:10)/10).
  1/10 mismatches
  x[9]: "#7A67EEF1"
  y[9]: "#7A67EEF2"

I guess there is a slight rounding or floating-point difference somewhere.