Ensure ASCII-armoured keyrings are text items.

Ensure ASCII-armoured keyrings are text items.

The gzip postprocess rule only applies to text items, so the
ASCII-armoured keyring is not compressed.

It does not seem possible to produce both binary and text items from a
filter depending on options, so we split the filter into two: one for
the binary export required by WKD and the other for the armoured export
for manual download.