change build job for build_requirements.py

[ru-fu]

Changing the build job to pre_install for the build_requirements.py script means that the build machine will already be fully set up, so that you can use pip to install any packages that are required in custom_conf.py and will otherwise give errors.

This is required to work around #197.

[dviererbe]

question: Can you explain how this solves #197?

note: My understanding of the problem is:

flowchart
  style dependency fill:red,color:white
  style requirements.txt stroke-dasharray: 2 2, fill: lightgrey
  style invisible stroke-width:0,fill:#00000000
  
  subgraph build_requirements.py
    direction TB
    invisible[" "] -->|"import"| custom_conf.py
    custom_conf.py -->|"import"| dependency["not yet installed module"]
  end

  build_requirements.py ---x|build fails| requirements.txt
  pip_install["pip install -r requirements.txt"] --x|"can not find"| requirements.txt

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I do not understand how calling build_requirements.py at pre_install changes the situation.

[ru-fu]

It doesn't solve the problem, but it makes it possible to work around it.

See here for an example.
The workaround is to "manually" install the required modules before running build_requirements.py.
However, if I try to do that at post_checkout, pip isn't installed yet, so it fails. Also if I try to explicitly install pip (as a command or with apt_packages), it isn't available at that stage.
It took me a lot of experimenting to figure out that pip IS available at pre_install. So if we change when build_requirements.py runs, we can easily pip-install packages before it runs.

[AnneCYH]

LGTM, thank you!