need_internal_tar: Check if we need to use R's internal tar implementation
In r-lib/pkginstall: Installs Packages from Local Files

This is slow, because we need to start an R child process, and the implementation is also very slow. So it is better to use an extranl tar program, if we can. We test this by trying to uncompress a .tar.gz archive using the external program. The name of the tar program is taken from the TAR environment variable, if this is unset then tar is used.