buildbot: fix coverage reports

A recent commit which added out-of-tree builds broke buildbot coverage
reports. This fixes them.

.buildbot

@@ -24,8 +24,8 @@ do_coverage()
     cd build
     ../configure --coverage
     make coverage-report
-    mkdir -p ./buildbot-upload/coverage/
-    cp -r ./build/coverage/html/* ./buildbot-upload/coverage/
+    mkdir -p ../buildbot-upload/coverage/
+    cp -r ./coverage/html/* ../buildbot-upload/coverage/
 }
 
 do_docs()