buildsys: import sa-update key from tarball

buildsys: import sa-update key from tarball

`make sa-updates.tgz` runs sa-update with a temporary directory and packs the
gathered rules into a tarball, which gets extracted during dpkg-buildpackage.

This patch creates a gpg-homedir and imports the sa-update-key distributed with
the upstream tarball, and changes the call to sa-update to use this directory
instead of the default '/etc/mail/spamassassin/sa-update-keys'.
In addition to being a bit more self-contained this also enables running
`make sa-updates.tgz` without being root.

The temporary gpg-homedir is added to '.gitignore'

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>