chore(deps): lock jekyll-sass-converter to v2.x on Linux-musl (cotes2020#829)

Also, make the division formula backward compatible with `jekyll-sass-converter 2.x` (cotes2020#848, cotes2020#849)

Ref: https://github.com/jekyll/jekyll-sass-converter#migrate-from-2x-to-3x

Author: cotes2020

Gemfile

@@ -17,3 +17,8 @@ end
 
 # Performance-booster for watching directories on Windows
 gem "wdm", "~> 0.1.1", :install_if => Gem.win_platform?
+
+# Lock jekyll-sass-converter to 2.x on Linux-musl
+if RUBY_PLATFORM =~ /linux-musl/
+  gem "jekyll-sass-converter", "~> 2.0"
+end

_sass/addon/commons.scss

@@ -841,7 +841,7 @@ $sidebar-display: "sidebar-display";
 
     @for $i from 1 through $tab-count {
       $offset: $tab-count - $i;
-      $top: -$offset * $tab-height + calc(($tab-height - $tab-cursor-height) / 2);
+      $top: calc(-#{$offset} * #{$tab-height} + (#{$tab-height} - #{$tab-cursor-height}) / 2);
 
       @if $i < $tab-count {
         > li.active:nth-child(#{$i}),