tree.less is not cached

Reproduction steps

1. visit http://localhost:8080/xwiki/bin/view/Sandbox/
2. open the browser debugger and make sure that "disable cache" is unchecked
3. click on the "Sandbox Test Page 1" link to go to http://localhost:8080/xwiki/bin/view/Sandbox/
4. observe the network tab

Expected

http://localhost:8080/xwiki/webjars/wiki%3Axwiki/xwiki-platform-tree-webjar/17.2.0-SNAPSHOT/tree.min.css?evaluate=true is cached

Actual

http://localhost:8080/xwiki/webjars/wiki%3Axwiki/xwiki-platform-tree-webjar/17.2.0-SNAPSHOT/tree.min.css?evaluate=true is not cached

Analysis

The stylesheet needs to be recomputed for each page because:

1. it relies on less variables for color theme
2. is relied on webjar script services to computed font-awesome icons
3. use of $xwiki.getSkinFile to get the spinner