Only RHS-formula accepted as lambda function

[teunbrand]

Thsi PR aims to fix #6384 and amends #6200.

Briefly, the reason bquote() input for scale names wasn't accepted is because it was interpreted as a two-sided formula.
This PR only attempts to cast a RHS-only formula as a function.

Reprex from issue:

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

eff <- "Efficiency"
ggplot(mtcars, aes(factor(cyl), mpg)) + 
  geom_boxplot() + 
  scale_y_continuous(name = bquote(.(eff)~(mi~gl^{-1})))

^{Created on 2025-03-28 with reprex v2.1.1}

[thomasp85]

LGTM