Chafa attempts sixel in unsupported cases

[m2Giles]

Chafa appears to attempt to use some sort of auto detection for determining to use sixels. I'm unsure if this is now default behavior or not. But when setting PV_TYPE=text in a sixel compatible terminal it appears to attempt to use sixel. However, this then fails in things like fzf.

For PV_TYPE=text in stpvimg, explicitly set the chafa format to symbols to avoid this behavior.