Fix the sidebar height

`calc` is parsed specially in SASS as if the interior is a plain unquoted string and requires using explicit interpolation syntax (see the "fun fact" at https://sass-lang.com/documentation/at-rules/function#plain-css-functions).

Author: Nemo157

templates/style/rustdoc-2021-12-05.scss

@@ -10,7 +10,7 @@
     .sidebar {
         margin-top: 0;
         top: $top-navbar-height;
-        height: calc(100vh - $top-navbar-height);
+        height: calc(100vh - #{$top-navbar-height});
         /* Since we use `overflow-wrap: anywhere;`, we never have the need for a X scrollbar... */
         overflow-x: hidden;