Github builders fails to download `isl-0.20` for crossbuild

[klensy]

[ALL  ]    --2021-10-06 09:39:56--  http://isl.gforge.inria.fr/isl-0.20.tar.xz
[ALL  ]    Resolving isl.gforge.inria.fr (isl.gforge.inria.fr)... 128.93.193.15
[ALL  ]    Connecting to isl.gforge.inria.fr (isl.gforge.inria.fr)|128.93.193.15|:80... failed: Connection timed out.
[ALL  ]    Retrying.

see:
#89495 (comment)
#89245 (comment)

and other latest build fails.

[klensy]

@rustbot label: +T-infra

[klensy]

see also: crosstool-ng/crosstool-ng#1609

[klensy]

@rustbot label: +P-high
as it prevents PR's from merges

[rustbot]

Error: Label P-high can only be set by Rust team members

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