DEV: Fix stylesheet manager flaky spec (PR #13846)

The cache_fullpath for the Stylesheet::Manager was the same for every test runner in a parallel test environment, so when other specs or other places e.g. the stylesheets_controller_spec ran rm -rf Stylesheet::Manager.cache_fullpath this caused errors for other specs running that went through the Stylesheet::Manager::Builder#compile path, causing the error

       No such file or directory @ rb_sysopen


Prior art for a fix like this:

This is where the spec is erroring:

And this is an example of another spec deleting the cache path: