[CI] Allow to configure a post build script

Summary:
This diff adds a post_build configuration element which allows for
running an arbitraty script as a step after all the targets are built.
Fixes the build for OSX.

Depends on D7408 and D7409.

Test Plan: Ran the builds locally, excepted IBD which I stopped before the end.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7403