substitute: Gracefully handle trailing slashes in URLs.

substitute: Gracefully handle trailing slashes in URLs.

Previously, using something like
"--substitute-urls=http://example.org///" would lead to a
'cache-narinfo!' call with #f as its second argument.

It would also do the wrong thing for URLs with a non-empty initial path
component, such as "http://example.org/foo/bar".

* guix/scripts/substitute.scm (fetch-narinfos)[handle-narinfo-response]:
Add call to 'basename' for PATH.