Revert "Reuse MARBLE_PLUGIN_INSTALL_PATH."

Note that this will break a cmake install when using a local
install path and QTONLY=TRUE, so needs a better approach in
the future. Workarounds are using sudo/root for make install
or disabling designer plugins (WITH_DESIGNER_PLUGIN=FALSE)

This reverts commit 2773f06a.

CCBUG: 327690