net_box.lua: fix "used variable with unused hint"

Since 0.26.0 luacheck emits a warning on the `_box` variable.
From luacheck v.0.26.0 release notes:

"Function arguments that start with a single underscore
get an "unused hint". Leaving them unused doesn't result
in a warning. Using them, on the other hand, is a
new warning (№ 214)."

Renamed `_box` to `__box`, which isn't considered unused.

Closes #7304.

NO_DOC=testing
NO_TEST=testing
NO_CHANGELOG=testing