Test rendering the vignette on mac and Windows in CI

[dylanhmorris]

Since the rendering of the pkgdown site confirms the vignette runs, we exclude running it from the Ubuntu R CMD check for efficiency, but we probably want to confirm it runs on mac and Windows.

[gvegayon]

Although I am all for rebuilding vignettes using different flavors of OS, I just want to ensure we all are aware of the option of sharing pre-built vignettes; see: this r-opensci article about pre-computed vignettes, the note in this section of the R pkgs book, and the section "Condition code in the code package on availability of the data package" of this paper about drat.

[seabbs]

We use pre-built vignettes in epinowcast etc with actions to keep them up to date. It works and is fine but its really annoying to have to store the figures etc in the repo so I would avoid if we can/for as long as possible

I wouldn't personally run the vignettes across platform and leave the unit tests for checking that but I don't think its harmful to do so.