Correctly render benchmark vignette in CRAN vs non-CRAN context

[RasmusSkytte]

Intent

Fixes #162

Approach

A PR on diseaystore (ssi-dk/diseasystore#185) fixed an issue with the data being plotted for the benchmarks on CRAN.
This PR implements these changes in the logic to ensure that when rendering on CRAN, we use the newest benchmarks.

In addition, the figures created on CRAN are the low resolution vignette figures that R uses for its vignettes.
These plots are adjusted so that they are readable now.

We therefore now first check if we are in a CRAN or non-CRAN context and then swap which benchmarks we plot and how they are plotted.

Example figures in a non-CRAN context (i.e. our website):

Example figures in a CRAN context (i.e. for the pre-rendered vignettes):

Known issues

N/A

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR

[RasmusSkytte]

This should be the last thing we need before we can release the next version of SCDB