[Buildbot] #3166: fix 'buildbot.net' build slave

[Buildbot trac]

#3166: fix 'buildbot.net' build slave
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Reporter:  sa2ajj  |      Owner:
    Type:  task    |     Status:  new
Priority:  major   |  Milestone:  sys - on-bb-infra
 Version:          |   Keywords:
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 The following builders are run on this build slave:

 * tarball-slave (success)
 * tarball-master (success)
 * linty (success)
 * docs (failed)
 * coverage (failed)
 * builds (failed)

 docs: `/bin/sh: 2: source: not found`.  it looks like it's looking for the
 virtual environment in the wrong place (why it worked before?)

 coverage: fails to create files in `/home/buildbot` (this is where it
 could put the results in the previous setup)

 builds: (this is strangest) can't find the `activate` script, though the
 relative path is OK

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Ticket URL: <http://trac.buildbot.net/ticket/3166>
Buildbot <http://buildbot.net/>
Buildbot: build/test automation