Add R packages to Singularity.rstudio

The added packages are often needed to build/install other packages.

rstudio/Singularity.rstudio

@@ -37,6 +37,4 @@ From: ubuntu:focal
   apt install -y ./rstudio-1.4.1717-amd64.deb
   apt install -y default-jre default-jdk
   R CMD javareconf
-  R -e "install.packages('rJava')"
-  R -e "install.packages('devtools')"
-  R -e "install.packages('Rcpp')"
+  R -e "install.packages('rJava', 'Rcpp', 'devtools')"