zbm-build.sh: don't upgrade packages when installing custom software

In cases where the `zfs` package has been updated since the container
image was built, this would trigger an upgrade of `zfs`; however, the
container does not include Linux headers by default, so the rebuild will
fail. Instead, skip the package upgrade. In the worst case, adding new
packages fails and the container image must be rebuilt.

releng/docker/zbm-build.sh

@@ -123,12 +123,11 @@ if [ -z "${ZBMTAG}" ]; then
 fi
 
 if [ "${#PACKAGES[@]}" -gt 0 ]; then
-  # Trigger an upgrade to make sure the package is installable
+  # Trigger a sync and upgrade to make sure the package is installable
   xbps-install -Syu xbps
-  xbps-install -Syu
 
   # Install all requested packages
-  xbps-install -y "${PACKAGES[@]}"
+  xbps-install -Sy "${PACKAGES[@]}"
 fi
 
 # shellcheck disable=SC2010