Linux: keybase does not use KDE download location

In the KDE settings, I have set the download folder to `~/Desktop`. And yet, keybase puts downloaded files into `~/Downloads`.

In my `~/.config/user-dirs.dirs`, it says
```
XDG_DOWNLOAD_DIR="$HOME/Desktop"
```
so it looks like keybase is not properly following XDG?

Start by tracing the Linux download-location handling in the Go client and compare it with the XDG setting in ~/.config/user-dirs.dirs. Reproduce the issue with XDG_DOWNLOAD_DIR set to "$HOME/Desktop" in KDE; done means downloaded files use that configured location instead of ~/Downloads.

Written by the indexing model from the issue text.