tests:openpgp: With gpgtar, extract tarball into an empty directory.

* tests/openpgp/defs.scm (setup-legacy-environment): Only
call create-gpghome in case of no --unpack-tarball.

At the tarball creation by setup.scm, it calls create-gpghome. So,
it's not needed in case of --unpack-tarball.

• GnuPG-bug-id: T8159
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>