build/debian: Remove old `vmlinuz` and `initrd.img` on x86_64

The existing `mv vmlinuz* vmlinuz` invocation errors out with `mv: target 'vmlinuz' is not a directory`when `vmlinuz` already exists from a previous run.

In this CL, we remove the old `vmlinuz` and `initrd.img` before replacing them with the extracted ones.

Bug: 381302852

Change-Id: I0ae2f937706c8a14bad68e9d871eb47273e603fa
Test: ./build_in_container.sh -a x86_64
diff --git a/build/debian/build.sh b/build/debian/build.sh
index 1e43b60..dfcffb6 100755
--- a/build/debian/build.sh
+++ b/build/debian/build.sh
@@ -238,6 +238,7 @@
 	)
 # TODO(b/365955006): remove these lines when uboot supports x86_64 EFI application
 elif [[ "$arch" == "x86_64" ]]; then
+	rm -f vmlinuz initrd.img
 	virt-get-kernel -a "${built_image}"
 	mv vmlinuz* vmlinuz
 	mv initrd.img* initrd.img