mkosi: stage volatile deb packages for upgrade test

Prepare scripts run before volatile packages are installed, so parse
the list from the config to ensure they are all included to avoid
failures due to some packages missing from the list.

Follow-up for 28e1f84d6a

mkosi/mkosi.conf.d/debian-ubuntu/mkosi.prepare

@@ -40,6 +40,7 @@ if [[ $deltaver -le 0 ]]; then
     exit 0
 fi
 
+declare -A packages=()
 distro_packages=()
 devel_packages=()
 
@@ -48,7 +49,18 @@ while IFS=$'\t' read -r package source package_architecture; do
     [[ $package_architecture == "$DISTRIBUTION_ARCHITECTURE" || $package_architecture == all ]] || continue
     [[ $package != *-dbgsym* ]] || continue
 
-    devel_packages+=("$package=$(dpkg-query --root="$BUILDROOT" --showformat='${Version}' --show "$package")")
+    packages["$package"]=
+done < <(dpkg-query --root="$BUILDROOT" --show --showformat='${Package}\t${source:Package}\t${Architecture}\n')
+
+# Prepare scripts run before VolatilePackages are installed, so include them explicitly.
+while read -r package; do
+    [[ $package != *-dbgsym* ]] || continue
+
+    packages["$package"]=
+done < <(jq --raw-output '.VolatilePackages[]' <"$MKOSI_CONFIG")
+
+for package in "${!packages[@]}"; do
+    devel_packages+=("$package=$develver")
 
     package_versions=$(apt-cache madison "$package")
     if awk -F '|' -v version="$distrover" '
@@ -63,7 +75,7 @@ while IFS=$'\t' read -r package source package_architecture; do
         ' <<<"$package_versions"; then
         distro_packages+=("$package=$distrover")
     fi
-done < <(dpkg-query --root="$BUILDROOT" --show --showformat='${binary:Package}\t${source:Package}\t${Architecture}\n')
+done
 
 if (( ${#distro_packages[@]} == 0 )); then
     echo "Skipping upgrade test, no distro packages found"