Since version 0.5.4.11, Xarchiver supports using either unrar or unar to
unpack .rar files. The former being released under an unfree license,
using the latter by default is preferable.
This avoids indirectly guarding the xarchiver package under
`nixpkgs.config.allowUnfree` due to the dependency on unrar.
Users who would like to use the unfree implementation may do so by
adding the unrar package to the environment from which they are calling
xarchiver.
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