GRNBoost2 not available in R 4.0? 

Also posted here - aertslab/GRNBoost#7, not sure which repository is more appropriate.

Anyway, trying to install (through Biocmanager) on Rstudio gives this error
```
Warning message:
package ‘GRNBoost2’ is not available (for R version 4.0.2) 
```
is this something users can circumvent (without reverting to R 3 and causing other incompatibility issues)?
