Improve update-cache.sh script

Requirements are now clear, simplified steps as well.

dist/rpm/update-cache.sh

@@ -1,20 +1,24 @@
-#! /bin/sh
+#!/usr/bin/bash
+
+# requirements:
+# bzip2 cpio perl-IO-Socket-SSL perl-Mojolicious
+# perl-Mojolicious-Plugin-AssetPack ruby3.3-rubygem-sass
 
 set -e
 
 export LC_ALL='en_US.UTF-8'
 export LANG='en_US.UTF-8'
-osc up
-rm -f _service\:*
-rm -f *.tar *.cpio
-osc service lr
-# special call for tar buildtime service
-osc service lr tar
 
-SD=$PWD
-cd MirrorCache
-tools/generate-packed-assets
-tar cvjf ../cache.tar.xz assets/cache assets/assetpack.db
+mkdir -p MirrorCache-update-cache
+rm -rf MirrorCache-update-cache/*
+pushd MirrorCache-update-cache
+
+cpio -id < ../MirrorCache-*.obscpio
+pushd MirrorCache-*
+
+./tools/generate-packed-assets
+tar cvjf ../../cache.tar.xz assets/cache assets/assetpack.db
 
-cd "$SD"
-osc up
+popd
+popd
+rm -rf MirrorCache-update-cache/*