Use newer gpg from sloppy

Else makebuildserver fails with:

==> default: The following additional packages will be installed:
==> default:   gpgconf libwebp6 libwebpdemux2 libwebpmux2 python3-cffi-backend
==> default:   python3-cryptography python3-idna
==> default: Suggested packages:
==> default:   python-cryptography-doc python3-cryptography-vectors python3-gssapi
==> default:   python-pil-doc python3-pil-dbg
==> default: Recommended packages:
==> default:   gnupg
==> default: The following NEW packages will be installed:
==> default:   libwebp6 libwebpdemux2 libwebpmux2 python3-cffi-backend python3-cryptography
==> default:   python3-idna python3-paramiko python3-pil
==> default: The following packages will be DOWNGRADED:
==> default:   gpg gpgconf
==> default: 0 upgraded, 8 newly installed, 2 downgraded, 0 to remove and 1 not upgraded.
==> default: W: chown to _apt:root of directory /var/cache/apt/archives/partial failed - SetupAPTPartialDirectory (1: Operation not permitted)
==> default: W: Not using locking for nfs mounted lock file /var/cache/apt/archives/lock
==> default: E: Packages were downgraded and -y was used without --allow-downgrades.
The SSH command responded with a non-zero exit status. Vagrant
assumes that this means the command failed. The output for this command
should be in the log above. Please read the output to determine what
went wrong.

Because at this time, stretch has gpg->gnupg 2.1.18-8deb9u4, stretch-backports has gpg 2.2.12-1+deb10u1bpo9+1 and stretch-backports-sloppy has gpg 2.2.20-1~bpo9+1.

This needs to be redo once we go to Buster, soon?