Load LESS variables via path traversal

Since these files are part of the same package, there is no need
to assume a Composer context to load these from. Instead, we can
just load them via the path relative to the current PHP file.

This assumption may break in certain environments, and it is
already broken when running (integration) tests.

src/Frontend/FrontendServiceProvider.php

@@ -72,8 +72,8 @@ public function boot()
 
     public function addBaseCss(SourceCollector $sources)
     {
-        $sources->addFile(base_path().'/vendor/flarum/core/less/common/variables.less');
-        $sources->addFile(base_path().'/vendor/flarum/core/less/common/mixins.less');
+        $sources->addFile(__DIR__.'/../../less/common/variables.less');
+        $sources->addFile(__DIR__.'/../../less/common/mixins.less');
 
         $this->addLessVariables($sources);
     }