n.breaks propagate to sec.axis

[teunbrand]

This PR aims to fix #4483.

Briefly, it captures n.breaks in a closure when copying the default breaks from a primary scale during secondary scale initialisation.

Reprex from issue:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

dat <- data.frame(x = 1:10, y = sample(x = 1:1000, size = 50, replace = TRUE)) 

ggplot(data = dat, aes(x = x, y = y)) +
  geom_point() +
  scale_y_continuous(
    n.breaks = 20,
    sec.axis = sec_axis(~.)
  )

^{Created on 2024-07-03 with reprex v2.1.0}

[thomasp85]

LGTM