fix(startup): init.lua: set $MYVIMRC to absolute path #15748

- main.c: remove os_setenv("MYVIMRC",…), it is already done by
  do_source().
  - This also sets $MYVIMRC to a full (absolute) path.
- code cleanup.

test/functional/core/startup_spec.lua

@@ -472,7 +472,7 @@ describe('user config init', function()
     clear{ args_rm={'-u' }, env=xenv }
 
     eq(1, eval('g:lua_rc'))
-    eq(init_lua_path, eval('$MYVIMRC'))
+    eq(funcs.fnamemodify(init_lua_path, ':p'), eval('$MYVIMRC'))
   end)
 
   describe 'with explicitly provided config'(function()