bugfix undefined border color variable for btn

scss/_buttons.scss

@@ -15,6 +15,7 @@
   --#{$prefix}btn-border-width: #{$btn-border-width};
   --#{$prefix}btn-border-color: transparent;
   --#{$prefix}btn-border-radius: #{$btn-border-radius};
+  --#{$prefix}btn-hover-border-color: transparent;
   --#{$prefix}btn-box-shadow: #{$btn-box-shadow};
   --#{$prefix}btn-disabled-opacity: #{$btn-disabled-opacity};
   --#{$prefix}btn-focus-box-shadow: 0 0 0 #{$btn-focus-width} rgba(var(--#{$prefix}btn-focus-shadow-rgb), .5);