fix: skip regular files on engine clean

[katafrakt]

This was found in a #570 , which added DETS file inside the cache directory. Then when engine clean was run, it failed because it could only handle directories.

The failure looked like this:

❯ ~/.local/bin/expert_darwin_arm64 engine clean --force
Kernel pid terminated (application_controller) ("{application_start_failure,xp_expert,{bad_return,{{'Elixir.XPExpert.Application',start,[normal,[]]},{'EXIT',{#{reason => enotdir,path => <<\"/Users/pawel.sw/Library/Caches/expert/hex.dets\">>,'__struct__' => 'Elixir.File.Error','__exception__' => true,action => <<\"list directory\">>},[{'Elixir.File','ls!',1,[{file,\"lib/file.ex\"},{line,2064}]},{'Elixir.XPExpert.Engine','-get_engine_dirs/0-fun-1-',3,[{file,\"lib/expert/engine.ex\"},{line,103}]},{'Elixir.Enum','-reduce/3-lists^foldl/2-0-',3,[{file,\"lib/enum.ex\"},{line,2520}]},{'Elixir.XPExpert.Engine',get_engine_dirs,0,[{file,\"lib/expert/engine.ex\"},{line,102}]},{'Elixir.XPExpert.Engine',clean_engines,1,[{file,\"lib/expert/engine.ex\"},{line,80}]},{'Elixir.XPExpert.Application',start,2,[{file,\"lib/expert/application.ex\"},{line,26}]},{application_master,start_it_old,4,[{file,\"application_master.erl\"},{line,299}]}]}}}}}")

Crash dump is being written to: erl_crash.dump...done

This PR makes the clean command skip any files in the cache directory.

[expert-backports]

Successfully created backport PR for v0.1:

• fix: skip regular files on engine clean #620