Filter out *-signed-template packages from kernel selection (#1104907)

When booting the installer with an amd64 netboot image in expert mode, linux-image-amd64-signed-template is wrongly included in the kernel selection.

I guess it is the same with arm64. riscv64 and loong64 are expected to have signed kernels in the future, so get ready.

Closes: #1104907