Revert to buildDir to fix F-Droid biuld

See:

https://monitor.f-droid.org/builds/log/ws.xsoh.etar/40#site-footer

Regression of: b8c2461

build.gradle

@@ -43,6 +43,6 @@ idea.project.settings {
 // External project configuration end
 
 tasks.register('clean', Delete) {
-    delete rootProject.layout.buildDirectory
+    delete rootProject.buildDir
     delete 'app/libs'
 }