Avoid invalid store-file-name by explicitly passing the destination
name, replacing any character not allowed in the store-file-name by an
underscore.
Fixes <http://issues.guix.gnu.org/issue/26175>
* guix/scripts/download.scm (safe-naensure-valid-store-file-nameme):
New function. (download-to-store*): Use it to generate a "safe"
basename of URL.