client: pxar: encode prelude based on writer variant

client: pxar: encode prelude based on writer variant

Currently, whether to encode the exlcude patterns passed via cli as
prelude or via the `.pxar-exclude-cli` is based on the presence of
a previous metadata accessor.
That leaves however to the encoding of the file entry instead of the
prelude for split archives in `data` mode and for the first snapshot
in a backup, creating undesired padding in the first payload chunk.

Therefore, use the pxar writer variant to make the decision instead.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>