ci: disable ccache when running CMake build system tests

Some tests check if certain files are rebuilt when source files are
'touch'ed. With ccache, 'touch'ing source files doesn't cause a
rebuild, hence the test fails. In case IDF_CCACHE_ENABLE was set in
the environment, unset it before starting the tests.

tools/ci/test_build_system_cmake.sh

@@ -35,6 +35,9 @@
 
 export PATH="$IDF_PATH/tools:$PATH"  # for idf.py
 
+# Some tests assume that ccache is not enabled
+unset IDF_CCACHE_ENABLE
+
 function run_tests()
 {
     FAILURES=