have LATEST-notmuch-<version>.tar.gz on releases web page

have LATEST-notmuch-<version>.tar.gz on releases web page

The notmuchmail/releases page used to have LATEST-notmuch-<version>
to link to the latest notmuch source tarball. This is confusing on
web page and on disk when the file has been downloaded. This change
looks a bit inconsistent with the 'rm' command just executed before.
$(TAR_FILE) is defined (currently) as $(PACKAGE)-$(VERSION).tar.gz;
as long as the prefix stays $(PACKAGE)-$(VERSION) and version begins
with a digit then this line is good in execution point of view.