Sandboxing local variables using setfenv?

This prints 5 and then 6. It's supposed to add 1 to any variable set, so a should be 6 and b should be 6, but __newindex doesn't affect a because it was declared locally. Is there any way to filter local variables?

do
    local e={print=print}
    setfenv(1,setmetatable({},{
        __index=function(t,k)
            return e[k]or loadstring('return '..k)()
        end,
        __newindex=function(t,k,v)
            e[k]=v+1
        end
    }))
end

local a=5
print(a)

b=5
print(b)