place kernel at /casper/vmlinux on riscv64

For no very good reason this is what debian-cd does, and what the grub
config isobuilder generates still expects. It probably makes sense to
switch to 'vmlinuz' like every other arch apart from ppc64el does but
for now I want to maintain compatibility with the old ISOs.

live-build/isobuilder/builder.py

@@ -268,7 +268,7 @@ class ISOBuilder:
             target.hardlink_to(src)
 
         kernel_name = "vmlinuz"
-        if self.arch == "ppc64el":
+        if self.arch in ("ppc64el", "riscv64"):
             kernel_name = "vmlinux"
 
         with self.logger.logged(