ci: test_build_system_cmake: ensure sdkconfig timestamp changes
ci: test_build_system_cmake: ensure sdkconfig timestamp changes

idf.py bootloader runs very quickly on some build servers, so when
sdkconfig is modified, the next idf.py invocation doesn't pick up the
timestamp difference. Add some delays to make sure the change is
noticed.

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf