Cross-compiling to i686-pc-linux-gnu on CentOS in the CI is mostly redundant with the ci/test/00_setup_env_i686_multiprocess.sh task (albeit it using clang):
One task seems sufficient as a sanity check, given that there seems to be no real demand for this architecture anyway.
Turning the task into a native one makes it possible to run the task natively on aarch64 or any other supported architecture.
Also, remove the install of the lbzip2 package, which is unused since commit a46065e36cf868265c909dc5edf29dc17be53c1f
Also, remove the CONFIG_SHELL env var, which is unused since the cmake migration. (CONFIG_SHELL in depends is still kept).