instead of hardwiring the toolchain image in github workflows, read it
from `tools/toolchain/image`. a dedicated reusable workflow is added to
read from this file, and expose its content with an output parameter.
also, switch iwyu.yaml workflow to this image, more maintainable this
way. please note, before this change, we are also using the latest
stable build of clang, and since fedora 40 is also using the clang 18,
so the behavior is not change. but with this change, we don't have
the flexibility of using other clang versions provided
https://apt.llvm.org in future.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#19655
