Dpkg::OpenPGP::Backend::GnuPG: Touch trustedkeys.gpg on temporary gpg home

The gpgv command expects a trustedkeys.gpg keyring, and if there is none
it will try the trustedkeys.kbx one, but then will emit an error such as:

  gpgv: unknown type of key resource 'trustedkeys.kbx'
  gpgv: keyblock resource '<GPGHOME>/trustedkeys.kbx': General error

This error will only show if the gpgv invocation failed for other
reasons, but it is still an unnecessary distraction. Avoid that error,
by touching the trustedkeys.gpg keyring.