Fix OMSimulator submodule update when directory layout changes

[AnHeuermann]

Issue

I changed the layout of OMSimulator submodules in OpenModelica/OMSimulator-3rdParty#98. This broke the library testing CI:

https://test.openmodelica.org/jenkins/blue/organizations/jenkins/LibraryTesting%2FLibraryTest/detail/LibraryTest/11099/pipeline/39/

CMake Error at 3rdParty/CMakeLists.txt:146 (add_subdirectory):
  The source directory

    /var/lib/jenkins/ws/OpenModelicaLibraryTestingWork/OMSimulator/3rdParty/libzip

  does not contain a CMakeLists.txt file.


CMake Error at 3rdParty/CMakeLists.txt:147 (add_library):
  add_library cannot create ALIAS target "oms::3rd::libzip" because target
  "zip" does not already exist.

Changes

Clean the working tree (including nested git repos) before running submodule update, so that directories that were replaced by submodules (or vice versa) don't block initialization.