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.

src/nvim/ex_cmds2.c

@@ -2112,7 +2112,7 @@ int do_source(char_u *fname, int check_other, int is_vimrc)
     verbose_leave();
   }
   if (is_vimrc == DOSO_VIMRC) {
-    vimrc_found(fname_exp, (char_u *)"MYVIMRC");
+    vimrc_found((char *)fname_exp, "MYVIMRC");
   }
 
 #ifdef USE_CRNL