geom_nodes() not sizing by nodal attributes

geom_nodes() doesn't seem to be sizing nodes properly when its fed sizes that are related to node attributes.

```r
## repex
data(flo)
n <- network(flo, directed = FALSE)
n%v%'labels' = paste(n%v%'vertex.names', sna::degree(n))

# each time i run this the size of each node changes
ggplot(n, aes(x, y, xend = xend, yend = yend)) +
geom_edges(colour = "steelblue") +
geom_nodes(size = degree(n), shape = 21, colour = "steelblue", fill = "white") +
geom_nodetext(aes(label = labels))+
theme_blank()
```

P.S. thank you for creating and sharing this package. It's my favorite network visualization tool.

Start with the supplied R reproduction using geom_nodes(size = degree(n)) and inspect how geom_nodes evaluates node attributes. The issue is resolved when repeated plots use stable, correctly sized nodes for the same degree values.

Written by the indexing model from the issue text.