The previous fix introduced in commit 8176277487 ("import: Avoid duplicate
trailing '/' in computed import URL.") regressed on the handling of *file*
source URIs:
$ guix refresh cairo --update
Starting download of /tmp/guix-file.xUKazf
From https://cairographics.org/cairo-1.18.4.tar.xz...
download failed "https://cairographics.org/cairo-1.18.4.tar.xz" 404 "Not Found"
* guix/gnu-maintenance.scm (import-html-release): Revert commit 8176277487,
instead adding a trailing slash only if it doesn't already exist.
Change-Id: I6e3889f14badd8843bbf6436ad62a1594f553afe
e77a676f21