Set toast z-index variable in the correct spot

The .toast-container tries to use the z-index CSS variable, which is defined under .toast. 
However, this variable is not accessible to the container. This change copies the variable to the spot where it can be used.

scss/_toasts.scss

@@ -38,6 +38,8 @@
 }
 
 .toast-container {
+  --#{$prefix}toast-zindex: #{$zindex-toast};
+
   position: absolute;
   z-index: var(--#{$prefix}toast-zindex);
   width: max-content;