ci: make sure the test language is set to en_US

For systems that have a different language, Neovim will use that
language in the ui. This can cause tests to fail because the output is
no longer the same (translations for messages in the ui are different).

scripts/minimal_init.lua

@@ -1,6 +1,8 @@
 -- Add current directory to 'runtimepath' to be able to use 'lua' files
 vim.cmd([[let &rtp.=','.getcwd()]])
 
+vim.cmd('language en_US')
+
 -- Set up 'mini.test' only when calling headless Neovim (like with `make test`)
 if #vim.api.nvim_list_uis() == 0 then
   -- Add 'mini.nvim' to 'runtimepath' to be able to use 'mini.test'