ci: fix luacheck config

vim must be a writable global (plugin sets vim.g, vim.bo, lsp
handlers); ignore unused self in method definitions.

Author: artur-shaik

.luacheckrc

@@ -1,8 +1,10 @@
 std = "luajit"
 cache = true
 codes = true
+self = false
 
-read_globals = {
+-- vim is writable: the plugin sets vim.g.*, vim.bo.* and lsp handlers
+globals = {
   "vim",
 }