When apt-get update fails, capture and display the failing mirror host to aid debugging. Adds grep-based extraction of the upstream host (from /etc/apt/sources.list.d/debian.sources or /etc/apt/sources.list) with a fallback to "unknown", and includes the host in the warning/echo. Applied to misc/build.func (inside pct exec), misc/install.func (pkg_update), and tools/pve/update-lxcs-cron.sh.
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