abstract pull repos logic for plugins to canvas-builds
abstract pull repos logic for plugins to canvas-builds

refs DE-424
flag = none

test-plan:
- Build passes
- Build fails if the pullRepos step fails

[canvas-builds-refspec=c5814b31bb8bb1f8f812def679654e458b5d295a]

Change-Id: I76ffd3d429c30876dd1897cd7307ed758e1bb7b7
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/255326
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Andrea Cirulli <b4e55ef5ee4bae8ad31f7c81009bfc0af1714a1f@instructure.com>
Reviewed-by: Aaron Ogata <11ccf682c06d1508642f943a3962a1c3d00b8e44@instructure.com>
Product-Review: Andrea Cirulli <b4e55ef5ee4bae8ad31f7c81009bfc0af1714a1f@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms