Update Illumos sysroot

[nikic]

@jclulow @pfmooney

The Illumos sysroot used by the dist-x86_64-illumos build is very outdated, dating back to 2018:

rust/src/ci/docker/scripts/illumos-toolchain.sh

Lines 50 to 55 in d9617c8

	SYSROOT_VER='20181213-de6af22ae73b-v1'
	SYSROOT_SUM='ee792d956dfa6967453cebe9286a149143290d296a8ce4b8a91d36bea89f8112'
	SYSROOT_TAR="illumos-sysroot-$SYSROOT_MACH-$SYSROOT_VER.tar.gz"
	SYSROOT_URL='https://github.com/illumos/sysroot/releases/download/'
	SYSROOT_URL+="$SYSROOT_VER/$SYSROOT_TAR"
	SYSROOT_DIR="$PREFIX/sysroot"

This needs to be updated to a newer version which at least includes the fix for https://www.illumos.org/issues/14418. LLVM 22 fails to build against the outdated Illumos sysroot, and given that the issue has been fixed upstream, I don't think we should work around this bug as we did in the past.

I don't see any newer sysroot releases on https://github.com/illumos/sysroot/releases, so I don't know how to do this myself. Are the sysroots published in some different place now?

[pfmooney]

Yes, sorry. This has been something we (I) have been meaning to get to, and have dropped the ball on.

[pfmooney]

I've raised it for discussion in the project. Hopefully it reaches quick consensus, and we'll create a much more recent sysroot for use here.