filter installations by GH_ORG

[hilmarf]

fixes: #782

[decyjphr]

The env variable GH_ORG is a probot configuration for manifest flow. That is not a required env variable so in most cases. it would not be set.

@hilmarf there might a different way to get the org in question. Let me investigate.

[Simon-Boyer]

@decyjphr Could we simply check if that variable (or even another one we define) is there and use it, if not we revert back to using installations[0]?

[renan-alm]

Right now this is a limitation.
This feature is needed for managing which orgs are in scope of the settings. Otherwise, we (users) are limited to use a single private GH App per org. In other words, it is a low-handing fruit.

The only work left on this PR is adding tests.