As of [1] goversioninfo supports specifying the icon to embed as part of
versioninfo.json. Use that new mechanism for consistency with specifying
other meta-data fields.
[1] https://github.com/josephspurrier/goversioninfo/pull/17
windres is not always present whereas goversioninfo is easy to install.
This also fixes the generation of resource.syso to be part of the
standard build script instead of only the AppVeyor CI build.
Finally, adapt update-version.sh to patch versioninfo.json.
