update_cache: use Rscript instead of littler.

update_cache: use Rscript instead of littler.

litter sets tempdir() to /tmp (or whatever is in one of the many TMP
environment variables) whereas Rscript is as R: tempdir() returns a
per-session temporary directory. Rscript seems more sane on this one.

Manifested as a bug:
cran2deb update
cran2deb build DPpackage
-> 404 no such package found in CRAN

-> but DPpackage in available.packages?!
-> available.packages using old cache (hence old version of package
metadata)
Rscript cleans up each time. littler doesn't.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/cran2deb@66 edb9625f-4e0d-4859-8d74-9fd3b1da38cb