From looking over the log it looks like the installation order is backwards (i.e. knitr is attempted to be installed before stringi) thus the failure to install knitr.
I think this issue may be helpful, in particular this suggestion. Perhaps there isn't a Windows binary available yet for stringi? You can try to force the type of package installation to be binary or source ONLY. This sounds like it could solve the installation order issue. Then if knitr installs, you should be able to build vignettes during the checks and the failure is solved. Or you could just add another step before installing other packages, forcing stringi to get installed first.
The last hacky solution to at least bypass the R CMD check error would be to add --no-build-vignettes to the rcmdcheck::rcmdcheck(args = ...) function. But this doesn't solve the root issue, which the above suggestion might.