pulling from the cache doesn't honor the fetchers setting

Summary

At least when pulling from the cache, bst doesn't seem to honor the fetchers setting.

Steps to reproduce

I have this in my config

scheduler:
  fetchers: 2
  builders: 2

and try to build something (e.g. from freedesktop-sdk).

What is the current bug behavior?

pulling from the cache uses 4 parallel jobs.

What is the expected correct behavior?

pulling from the cache should use 2 parallel jobs.

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.2

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