add makefile prefix, custom reqs cache key args

[matt-codecov]

needed for codecov/umbrella#30

previously i added working_directory to these workflows so that umbrella could make each one run cd apps/worker or similar at the beginning. it turns out, however, that we need to build requirements images in the umbrella root rather than apps/worker etc. so this PR replaces working_directory with a new make_target_prefix argument: if the workflow ordinarily calls make build, umbrella can use this argument to make it call make worker.build

unfortunately, i couldn't completely get rid of the old working_directory. with the way things are set up, things like app.tar or junit.xml are still dumped in apps/worker. so i renamed the argument output_directory to match its new function

finally, this PR adds the reqs_cache_key argument to build-app.yml and self-hosted.yml. this allows umbrella to pass in a special cache key for its special requirements images which include an extra hash. if the argument isn't provided, the cache key is computed like normal.