Elementary: form a vector via c(...):
install.packages(c("EIAdata", "gdata", "ggmap", "ggplot2")) # rest omitted
so that you have one first argument of length > 1.
Personally, I prefer install.r from littler so I’d do (at the Unix command-line):
install.r EIAdata gdata ggmap ggplot2 # rest omitted again
Note that there is no limit to the number of arguments. It was just easier for me to write this with four packages than the 20-some from your example.