Set publicSourceBranch var for matrix generation

[mthalman]

The changes from #5942 caused a regression in GenerateBuildMatrix job for this repo. It failed in the Set Image Info Path Vars step with this error:

   5 |  $publicSourceBranch = "$(publicSourceBranch)"
     |                           ~~~~~~~~~~~~~~~~~~
     | The term 'publicSourceBranch' is not recognized as a name of a cmdlet,
     | function, script file, or executable program. Check the spelling of the
     | name, or if a path was included, verify that the path is correct and try
     | again.

Example build (internal link)

This occurs because this repo has special logic for setting the publicSourceBranch variable that wasn't being executed for the GenerateBuildMatrix job. It is set in a step template that needs to be passed in as custom init steps to the job, just as it is done for build and other jobs.

These changes include changes to eng/common that I'll backport to docker-tools.