revdep FIESTA new failures to re-build vignettes

[tdhock]

revdep FIESTA uses data.table in vignettes, which fail to rebuild when data.table github master is installed.

Quitting from lines 220-224 (FIESTA_tutorial_SA.Rmd) 
Error: processing vignette 'FIESTA_tutorial_SA.Rmd' failed with diagnostics:
Incompatible join types: x.CONDID (integer) and i.DOMAIN (character)

and

Quitting from lines 218-224 (FIESTA_tutorial_MA.Rmd) 
Error: processing vignette 'FIESTA_tutorial_MA.Rmd' failed with diagnostics:
Column or expression 1 of 'by' or 'keyby' is type 'NULL' which is not currently supported. 
If you have a compelling use case, please add it to https://github.com/Rdatatable/data.table/issues/1597. 
As a workaround, consider converting the column to a supported type, 
e.g. by=sapply(list_col, toString), whilst taking care to maintain distinctness in the process.

git bisect says this error started happening after merging #5084
Several other revdeps broke after merging that same PR, and there is a proposed follow-up #5133
Not sure if it fixes this issue though.