Mix progressbar color from alerted text fg

The previous approach was not really harmonic and becomes worse for other color
combinations for normal and alerted text.

source/beamercolorthememetropolis.dtx

@@ -158,7 +158,7 @@
 \setbeamercolor{progress bar}{%
   use=alerted text,
   fg=alerted text.fg,
-  bg=normal text.bg!50!normal text.fg
+  bg=alerted text.fg!50!black!30
 }
 \setbeamercolor{title separator}{
   use=progress bar,