Run tests with gpg1.

Add build dependency on gnupg1 and set CI_GPG in debian/rules and
debian/tests/pkg-perl/smoke-env.

The tests are skipped if no gpg is found but if gnupg2 is installed they
fail because of the key material in t/assets/gpg.

CPAN RT#112569 provides a different solution: to convert the key material to
gnupg2 before running the tests. This works as well but leaves more changed
files behind.

Thanks: Lucas Nussbaum for the bug report.
Closes: #835711