PackageInfo: Explicitly trust Ubuntu and Debian keyrings.

The debian-archive-keyring package on Ubuntu doesn't automatically add its
keys to APT's trust keyring, likewise for the Ubuntu package on Debian.

PackageInfo/update_apt

@@ -59,6 +59,8 @@ update_apt() {
         -o="Dir::State::Status=$DIR/$DIST.status" \
         -o="Dir::Etc::SourceList=$DIR/$DIST.list" \
         -o="Dir::Etc::SourceParts=''" \
+        -o="Dir::Etc::trusted=/usr/share/keyrings/debian-archive-keyring.gpg" \
+        -o="Dir::Etc::trusted=/usr/share/keyrings/ubuntu-archive-keyring.gpg" \
         -o="Dir::Cache=$DIR/cache" \
         update
 }