github-ci: add more cleanup for running WS

Found that 'actions/checkout' does not remove all temporary files from
previous runs in submodules [1], it runs only 'git clean --xffd' [2]:

  libgcov profiling error:/home/ubuntu/actions-runner/_work/tarantool/tarantool/src/lib/small/CMakeFiles/small.dir/small/small_class.c.gcda:overwriting an existing profile data with a different timestamp

To avoid of it added:

  git submodules foreach --recursive 'git clean -xffd'

to 'actions/environment' which is run after each 'actions/checkout'.

Part of #5986

[1]: https://github.com/tarantool/tarantool/runs/2318244478?check_suite_focus=true#step:5:4012
[2]: https://github.com/actions/checkout/issues/358