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:

ggplot2/R/backports.R

Lines 55 to 56 in ddd207e

	# Unavailable prior to R 4.1.0
	linearGradient <- version_unavailable

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.

[thomasp85]

LGTM