d/r-base-core.postinst: Set default papersize with sed instead of perl (Closes: #1028651)

Gioele Barabuccirequested to merge
gioele/r-base:no-perl-maintscripts into master

r-base-core.postinst uses Perl to set $papersize as a default fallback in Renviron.

Such simple processing can be achieved also with a very similar sed invocation.

Using sed instead of Perl reduces the number of packages required to install r-core-base. (Perl is a candidate for removal from essential, see https://wiki.debian.org/Proposals/EssentialOnDiet.)

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