perl is a run-time dependency, so it should be in buildInputs rather
than nativeBuildInputs.
This has been preventing patchShebangsAuto() in fixupOutputHooks from
patching the /usr/bin/perl interpreter shebang in $out/bin/rrsync since
61bc03c017.
Resolves#71198
Extract the rsync source fetching into its own expression and use that
expression to fetch the same source for rsync and rrsync.
rrsync is just copied from the support folder of rsync, no configure or build
needed. Also none of the rsync patches are needed. Only the path to rsync needs
to be patched into rrsync.
