Fix bug in fallback of `grid::linearGradient()`

[teunbrand]

This PR does not fix an active issue. Instead it fixes a bug that should have been an issue, but isn't.

Briefly, when linearGradient is found in {grid}'s namespace, we want to declare that the linearGradient() function that ggplot2 uses is the one from {grid} and not the one we define here:

https://github.com/tidyverse/ggplot2/blob/ddd207e926cc1c1847dc661d9a099b8ec19c4010/R/backports.R#L55-L56

However, we're assigning the result of linearGradient(), not the function itself. This PR rectifies this mistake. This doesn't really affect anybody as when we use linearGradient() as a function, the {grid} version is used anyway because ggplot2::linearGradient is not a function.