Auto merge of #89661 - rusticstuff:libisl_mirror_take_two, r=Mark-Simulacrum

Auto merge of #89661 - rusticstuff:libisl_mirror_take_two, r=Mark-Simulacrum

CI: Use mirror for libisl downloads for more docker dist builds

http://isl.gforge.inria.fr fell from the net a couple of days ago. It hosts libisl source tarballs required by crosstool-ng, which we use for our docker dist cross-compilation builds. Some of the affected builds were already fixed in #89599.

This PR sets a mirror URL for the other builds requiring libisl-0.14. They use an older version of crosstool-ng (1.22.0), which has only one mirror setting for all downloads.

r? `@Mark-Simulacrum`