build: fetch kernel from cdn.kernel.org static mirror

Author: toksdotdev

Makefile

@@ -1,6 +1,8 @@
 KERNEL_VERSION = linux-6.12.96
-# Fetch stable snapshots directly so HTTP errors are visible instead of cached as tarballs.
-KERNEL_REMOTE = https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/snapshot/$(KERNEL_VERSION).tar.gz
+# Static release tarball from the kernel.org CDN: reliable and cached, unlike the
+# git.kernel.org cgit snapshot endpoint, which regenerates a ~250MB tarball per
+# request and 502s under CI load.
+KERNEL_REMOTE = https://cdn.kernel.org/pub/linux/kernel/v6.x/$(KERNEL_VERSION).tar.gz
 KERNEL_TARBALL = tarballs/$(KERNEL_VERSION).tar.gz
 KERNEL_SOURCES = $(KERNEL_VERSION)
 KERNEL_PATCHES = $(shell find patches/ -name "0*.patch" | sort)