Fix R CMD CHK on 3.x R versions

[mbjones]

We are seeing failures in the tests for older versions of R on ubuntu through CI tests because the suggested package taxadb is not available. The error is: Error: Package suggested but not available: ‘taxadb’.

Discussing with @cboettig on slack, he said:

taxadb is just a Suggested package, requiring R 4.x would be too much. Uwe at CRAN suggested we switch taxadb from rappdirs to tools::R_user_dir() because it plays nicer with CRAN policies (but was only added to tools in R > 4.0)

adding a check for skipping those checks on earlier R versions would be best I think

So, the proposal is to skip those checks for now on CRAN. Alternatively, we could switch to using tools::R_user_dir() but only if we are willing to increase our minimum version to R > 4.0.

[cboettig]

done in #321