Test skip due to different vdiffr version

[kyleam]

This is a follow-up to gh-18. On Metworx, I hit into a skip that looks like this:

Skip (test-base-plot.R:284:5): Base plots: Plot alignment with different saving to different sizes [PMF-PLOT-024]
Reason: SVG snapshot generated under a different vdiffr version.
Please update your snapshots.

This same skip is seen on a test build with the r-dev-ci-mrgval image.

@seth127, on the other hand, isn't seeing this skip, and packageVersion("vdiffr") reports matching versions for us, so it's unclear what's going on here.