This fixes a bug where calling `ctest_submit()` would result in a Done.xml
file with an empty <buildId> element.

CDash responds with a buildId when CTest submits a file. For this reason,
Done.xml needs to generated after CTest has submitted some other file to
CDash. This change also has the benefit of making Done.xml's timestamp as
late as possible, giving us a more accurate record of how long the entire
build took to complete.