Duplicate task slugs when creating more than one WorkerVersion build task

With a repo YAML config that states there are multiple workers that use different Dockerfiles:

---
version: 2
type: worker
workers:
  - slug: worker1
    name: blah1
    type: lol
    docker:
      build: Dockerfile1
  - slug: worker2
    name: blah2
    type: lol
    docker:
      build: Dockerfile2

The import_git.worker task will attempt to create two build tasks, both named docker_build_worker2. This causes the second task, for Dockerfile2, to not be created and instead have a HTTP 500 (ponos#111).

This line is the only place where a worker is assigned, in a first for loop; one the loop is completed, the last worker remains in this variable, and in the next loop to create one task per Dockerfile, only the last worker's slug will be used.

Since multiple workers could share the same Dockerfile, naming the build task after the worker's slug does not make much sense and it might be better to name the tasks after the Dockerfile names themselves.