Log Message:
If LOWER_OPSYS is "darwin", don't pass this as the OS type as this
package unsurprisingly has no idea what a "darwin" is. Instead use
"netbsd" in the hopes that the netbsd config is adequate for all
pkgsrc use. (If this doesn't work, try "macosx".)

I'm going to continue passing the contents of LOWER_OPSYS everywhere
else, because there's some chance it might actually be working as is
and we're in a freeze.

Might fix PR 37513.