fix(kernel): fall back to linux-lts where linux-virt is absent

Alpine does not build the linux-virt flavor for riscv64, so --kernel
without a path failed there with 'linux-virt package not found'. Scan
the APKINDEX once for both flavors and prefer linux-virt, taking
linux-lts when the virt flavor is missing.

Also store the cached kernel decompressed: riscv64 and aarch64 ship
their image gzipped (Image.gz), and QEMU's riscv -kernel loader
understands only ELF, uImage and raw images, so the gzipped image hung
at boot after the OpenSBI banner.

crates/ecr-cli/src/cli.rs

@@ -36,7 +36,8 @@ pub struct Args {
     /// Boot with QEMU system emulation (optionally specify kernel path with =PATH, or omit to download default)
     ///
     /// Examples:
-    ///   --kernel             Download and use the default Alpine linux-virt kernel
+    ///   --kernel             Download and use the default Alpine kernel
+    ///                        (linux-virt, or linux-lts on riscv64)
     ///   --kernel=./vmlinuz   Use a specific kernel file
     ///
     /// The kernel path must use `=` syntax: with `--kernel ./vmlinuz` the