Do not produce a bad output if system() sets errno.

For some reason this seems necessary on my machine,
otherwise `os.execute()` is triggering the wrong results,
caused by `system()` setting `errno`. This might have
to do with my updated build of Glibc. In any case this
patch should be harmless.

diff -Nur lua-5.4.7.old/src/loslib.c lua-5.4.7/src/loslib.c
--- lua-5.4.7.old/src/loslib.c	2024-06-13 19:15:09.000000000 -0300
+++ lua-5.4.7/src/loslib.c	2024-08-10 22:14:04.164723189 -0300
@@ -142,8 +142,8 @@
 static int os_execute (lua_State *L) {
   const char *cmd = luaL_optstring(L, 1, NULL);
   int stat;
-  errno = 0;
   stat = l_system(cmd);
+  errno = 0;
   if (cmd != NULL)
     return luaL_execresult(L, stat);
   else {