Flu vaccine lasts 6 months instead of forever (#40287)

* Make flu shot last 6 months instead of forever

The add_effect method's fourth argument "permanent" was set to true,
making the flu shot last forever, despite the 30_days duration. This
commit removes the permanent flag, and changes duration to 6 months (24 weeks).

src/iuse.cpp

@@ -835,7 +835,7 @@ int iuse::flu_vaccine( player *p, item *it, bool, const tripoint & )
 {
     p->add_msg_if_player( _( "You inject the vaccine." ) );
     p->add_msg_if_player( m_good, _( "You no longer need to fear the flu, at least for some time." ) );
-    p->add_effect( effect_flushot, 30_days, num_bp, true );
+    p->add_effect( effect_flushot, 24_weeks, num_bp, false );
     p->mod_pain( 3 );
     item syringe( "syringe", it->birthday() );
     p->i_add( syringe );