sanity: Check if tar is gnutar

In sanity.bbclass the tar version is checked as tar needs to be recent enough
for reproducible builds. Tar could also be provided by other means then gnutar,
but we mean the version of gnutar in the check. Hence we also should ensure
that the installed tar is gnutar.

[YOCTO #14205]

Signed-off-by: Simone Weiß <simone.p.weiss@posteo.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>