ci: make LuaJIT integration workflow rerunnable

If Tarantool tests fail in LuaJIT integration workflow used in
tarantool/luajit repository and LuaJIT submodule is already bumped in
tarantool/tarantool repository, one can't rerun the failed job since
"LuaJIT bump" step brings down the whole pipeline with "nothing to
commit" reason.

There are still many flaky tests, so to make this integration workflow
rerunnable from tarantool/luajit repository --allow-empty option is
added to the git commit command.

NO_DOC=ci
NO_TEST=ci
NO_CHANGELOG=ci

Reviewed-by: Yaroslav Lobankov <y.lobankov@tarantool.org>
Reviewed-by: Sergey Kaplun <skaplun@tarantool.org>
Signed-off-by: Igor Munkin <imun@tarantool.org>