Patterns in geom_sf()

[teunbrand]

This PR aims to fix #5716.

Briefly, it fixes a small oversight where we forgot to switch from alpha() to fill_alpha() in #5299.
Reprex from issue:

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

grad <- grid::linearGradient(c("red", "black"))

nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)
ggplot(nc) +
  geom_sf(fill = grad)

^{Created on 2024-02-27 with reprex v2.1.0}

[thomasp85]

LGTM