Fix keyring copy on Debian 13/trixie

It seems as though the `/usr/share/keyrings/debian-archive-keyring.gpg`
on Debian 13/trixie is actually a symlink to
`debian-archive-keyring.pgp` in the same directory. This means that the
`cp -a` in `mkimage` copies the symlink and not the actual keyring,
resulting in the following error:

    E: specified keyring file (/<tmpdir>/debian-archive-keyring.gpg) not found

Using `cp -aL` ensures the file that is symlink source actually gets
copied.

Author: jerrykan

mkimage

@@ -17,7 +17,7 @@ exec 2> >(tee -ia "$LOGFILE" >&2)
 
 DEBOOTSTRAP_DIR=$(mktemp -d)
 cp -a /usr/share/debootstrap/* "$DEBOOTSTRAP_DIR"
-cp -a /usr/share/keyrings/debian-archive-keyring.gpg "$DEBOOTSTRAP_DIR"
+cp -aL /usr/share/keyrings/debian-archive-keyring.gpg "$DEBOOTSTRAP_DIR"
 cp -a "${ROOT}/debootstrap/"* "${DEBOOTSTRAP_DIR}/scripts"
 
 KEYRING=$DEBOOTSTRAP_DIR/debian-archive-keyring.gpg