ggh4x: Rotate group labels with guide_axis_nested ggplot2

Hello I am using this package I found very handy

So this is the data:

df <- data.frame(
  x = LETTERS[1:16],
  group = rep(c("Group 1", "Group 2", "Group 3"), c(5, 3, 8)),
  value = rpois(16, 10)
)

And this is the code:

ggplot(df, aes(paste0(x, "&", group), value)) +
  geom_col() +
  guides(y = ggh4x::guide_axis_nested(delim = "&")) + coord_flip

This produces this plot:

How can I change the group labels (Group 1, Group 2, and Group 3) to be rotated 90 degrees to the left? Just the group labels.

To style the nested parts of the axis the ggh4x package provides the theme elements ggh4x.axis.nestline.x/y and ggh4x.axis.nesttext.x/y, i.e. to rotate the group labels you could use ggh4x.axis.nesttext.y like so

library(ggplot2)
library(ggh4x)

set.seed(123)

ggplot(df, aes(paste0(x, "&", group), value)) +
  geom_col() +
  guides(y = ggh4x::guide_axis_nested(delim = "&")) +
  coord_flip() +
  theme(
    ggh4x.axis.nesttext.y = element_text(
      angle = 90, hjust = .5
    )
  )