Description: Use permanent link for source download instead of quickly-deleted "most-recent" link

Imagemagick provides multiple download links for the source archive:

From PKGBUILD: https://imagemagick.org/download/$_tarname.tar.xz{,.asc}
Permalink e.g.: https://download.imagemagick.org/ImageMagick/download/releases/ImageMagick-7.1.0-8.tar.xz

Proposed new PKGBUILD entry in sources: https://download.imagemagick.org/ImageMagick/download/releases/$_tarname.tar.xz{,.asc}


While this currently works for the most recent version, using the permalink is a quality of life improvement if one e.g. wants to download previous source versions directly via makepkg.

We ran across this while trying to download imagemagick sources via a script for version 7.1.0-7, which we use in a finished project (so upgrading to a more recent version was out of the question). The PKGBUILD was not able to download this older version without tweaks.


Pros:
* Works for all releases of ImageMagick on their download mirror
* Works exactly the same for the most recent version

Cons:
* none.


Additional info:
* package version: 7.1.0-8

Steps to reproduce:
* works with most recent version, breaks e.g. with 7.1.0-7