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.

scripts/Dpkg/OpenPGP/Backend/GnuPG.pm

@@ -213,6 +213,9 @@ sub _gpg_verify {
     my @exec;
     if ($self->{cmdv}) {
         push @exec, $self->{cmdv};
+        # We need to touch the trustedkeys.gpg keyring, otherwise gpgv will
+        # emit an error about the trustedkeys.kbx file being of unknown type.
+        file_touch("$gpg_home/trustedkeys.gpg");
     } else {
         push @exec, $self->{cmd};
         push @exec, qw(--no-options --no-default-keyring --batch --quiet);