ci: add nightly CI job to build CMake in-place

An in-place (in-source) CMake nightly build has been running outside CI for years. It covers both building CMake in-place, and running many of the test cases in-place. Occasionally it catches problems not visible in out-of-source builds. Move this job into CI as part of our nightly pipeline.

Also rename ctest_test_external.cmake to ctest_standalone.cmake. The script is a standalone ctest script that runs all steps. It can be used for more than just external tests.