More robust checking of age of apt cache (#90)

install_and_cache_pkgs.sh

@@ -45,12 +45,11 @@ log "done"
 log_empty_line
 
 log "Updating APT package list..."
-last_update_delta_s=$(($(date +%s) - $(date +%s -r /var/cache/apt/pkgcache.bin)))
-if test $last_update_delta_s -gt 300; then
+if [[ -z "$(find -H /var/lib/apt/lists -maxdepth 0 -mmin -5)" ]]; then
   sudo apt-fast update > /dev/null
   log "done"
 else
-  log "skipped (fresh by ${last_update_delta_s} seconds)"
+  log "skipped (fresh within at least 5 minutes)"
 fi
 
 log_empty_line