Set math.randomseed before generating sponsor message

As explained in the docstring to `nfnl.core/rand`:

> You must have a random seed set before running this:
> `(math.randomseed (os.time))`

At present, Conjure does not do this and as a result the sponsor message
is not selecting a random sponsor (unless the user happens to call
`math.randomseed` within their Neovim environment).

This commit adds the call recommended in the docstring.

Author: pyrmont

fnl/conjure/log.fnl

@@ -53,6 +53,7 @@
     (vim.treesitter.stop buf)
     (tset vim.bo buf :syntax "on"))
 
+  (math.randomseed (os.time))
   (vim.api.nvim_buf_set_lines
     buf 0 -1 false
     [(str.join [(client.get :comment-prefix)