config/debian-security: set GnuPG homedir for generate-releases

See 3304b36a

config/debian-security/dak.conf

 Dinstall
 {
    // was non-us.d.o path before
+   SigningHomedir "/srv/security-master.debian.org/s3kr1t/dot-gnupg";
    SigningKeyring "/srv/security-master.debian.org/s3kr1t/dot-gnupg/secring.gpg";
    SigningPubKeyring "/srv/security-master.debian.org/s3kr1t/dot-gnupg/pubring.gpg";
    SendmailCommand "/usr/sbin/sendmail -odq -oi -t";