Fix the absolute path to the latest ISO

scripts/build_in_archiso_vm.sh: 8d96297d introduced a regression by which the filename of the downloaded latest ISO would still contain the URL path. Populate the iso variable with the temporary directory plus the remote filename for the iso instead.

scripts/build_in_archiso_vm.sh

@@ -121,7 +121,7 @@ prepare_boot() {
       printf 'Error: Could not find latest ISO\n'
       exit 1
     else
-      iso="${PWD}/${_latest_iso##/*}"
+      iso="${tmpdir}/archlinux-${arch}.iso"
     fi
   fi