test: Always use the latest available kernel from Alpine

When the kernel package is updated, the old versions are removed. Thus
we have to locate the current version via the index, rather than
relying on a fixed version.

test/Makefile

@@ -11,16 +11,13 @@ host-aarch64 := aarch64
 host-x86_64  := x86_64
 
 al = alpine-minirootfs-3.20.2-$(1)
-lx = linux-virt-6.6.47-r0
 tc = $(tc-arch-$(1))--musl--bleeding-edge-2024.05-1
 
 al-repo := https://dl-cdn.alpinelinux.org/alpine/v3.20
 tc-repo := https://toolchains.bootlin.com/downloads/releases/toolchains
 
 al-tar = $(call al,$(1)).tar.gz
 al-url = $(al-repo)/releases/$(1)/$(call al-tar,$(1))
-lx-tar = $(lx).apk
-lx-url = $(al-repo)/main/$(1)/$(lx-tar)
 tc-tar = $(call tc,$(1)).tar.xz
 tc-url = $(tc-repo)/$(tc-arch-$(1))/tarballs/$(call tc-tar,$(1))
 
@@ -108,11 +105,21 @@ cache/$(call al-tar,$(1)): |cache
 
 # Kernel
 
-work/$(1)-rootfs/boot/vmlinuz-virt: cache/$(lx)-$(1).tar.gz |work/$(1)-rootfs
-	tar -C work/$(1)-rootfs -maxf $$< 2>/dev/null
-
-cache/$(lx)-$(1).tar.gz: |cache
-	wget -O $$@ $(call lx-url,$(1))
+work/$(1)-rootfs/boot/vmlinuz-virt: |work/$(1)-rootfs
+	wget -O- $(al-repo)/main/$(1)/APKINDEX.tar.gz \
+		| gunzip \
+		| tar -Ox APKINDEX \
+		| grep -A1 -E "^P:linux-virt$$$$" \
+		| awk -F: '$$$$1 == "V" { printf("linux-virt-%s", $$$$2); }' \
+		>cache/lx-$(1)
+
+	[ -f cache/$$$$(cat cache/lx-$(1))-$(1).tar.gz ] \
+		|| wget \
+			-O cache/$$$$(cat cache/lx-$(1))-$(1).tar.gz \
+			$(al-repo)/main/$(1)/$$$$(cat cache/lx-$(1)).apk
+
+	tar -C work/$(1)-rootfs -maxf \
+		cache/$$$$(cat cache/lx-$(1))-$(1).tar.gz 2>/dev/null
 
 
 # Toolchain