[CI] Introduce assign-build:getBuildID
Summary: Extracts buildID from buildInfo if it exists.
Test Plan: Got buildID as expected.
Reviewers: ashoat, palys-swm
Reviewed By: ashoat
Subscribers: palys-swm, Adrian, karol-bisztyga, benschac, boristopalov
Differential Revision: https://phabricator.ashoat.com/D2371