[FTBFS] gnupg2 ftbfs when writing to $HOME on tests

Currently gnupg2 version 2.1.15-1ubuntu7 FTBFS because the opengpg tests try to write into $HOME.

https://launchpadlibrarian.net/337966140/buildlog_ubuntu-artful-amd64.gnupg2_2.1.15-1ubuntu7_BUILDING.txt.gz

Making check in openpgp
make[3]: Entering directory '/<<PKGBUILDDIR>>/build/tests/openpgp'
LC_ALL=C EXEEXT= PATH=../gpgscm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games TMP=/tmp srcdir=/<<PKGBUILDDIR>>/build/../tests/openpgp objdir=/<<PKGBUILDDIR>>/build GPGSCM_PATH=/<<PKGBUILDDIR>>/build/../tests/gpgscm:/<<PKGBUILDDIR>>/build/../tests/openpgp /<<PKGBUILDDIR>>/build/tests/gpgscm/gpgscm \
  run-tests.scm version.scm mds.scm decrypt.scm decrypt-dsa.scm sigs.scm sigs-dsa.scm encrypt.scm encrypt-dsa.scm seat.scm clearsig.scm encryptp.scm detach.scm detachm.scm armsigs.scm armencrypt.scm armencryptp.scm signencrypt.scm signencrypt-dsa.scm armsignencrypt.scm armdetach.scm armdetachm.scm genkey1024.scm conventional.scm conventional-mdc.scm multisig.scm verify.scm gpgv-forged-keyring.scm armor.scm import.scm ecc.scm 4gb-packet.scm tofu.scm gpgtar.scm use-exact-key.scm default-key.scm export.scm ssh.scm issue2015.scm issue2346.scm issue2417.scm issue2419.scm
gpg: Fatal: can't create directory '/sbuild-nonexistent/.gnupg': No such file or directory

FAIL: version.scm
gpg: Fatal: can't create directory '/sbuild-nonexistent/.gnupg': No such file or directory

FAIL: mds.scm
gpg: Fatal: can't create directory '/sbuild-nonexistent/.gnupg': No such file or directory

FAIL: decrypt.scm
gpg: Fatal: can't create directory '/sbuild-nonexistent/.gnupg': No such file or directory

One fix is to force GNUPGHOME to point inside the build directory, another option is to upgrade it to a newer release that modified the tests to remove some directory dependencies [1]. The upgrade requires a merge from Debian, creates new packages and changes how gnupg2 interacts with systemd, thus not ideal so close to the release.

[1] https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg.git;a=commit;h=7e19786a5ddef637d1d9d21593fecf5a36b6f372