Fix publishing previous-image
Fix publishing previous-image

Fixes: CCI-283

flag = none

Test plan:
- PATCHSET_TAG and PUBLISHABLE_TAG should match in the print env step
- observe that the canvas-lms:previous-image steps work as intended on
  subsequent builds

Change-Id: I7a5da8703b118dc6a1d0bb209056235039156c30
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/229910
Reviewed-by: Jacob Powell <03c892efa7d90f6161831192fc2bb1ab9ff2d4d9@instructure.com>
Reviewed-by: James Butters <4c5a489c9bb72d41f20b3b22aaf4c9b864e32f20@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: James Butters <4c5a489c9bb72d41f20b3b22aaf4c9b864e32f20@instructure.com>
Product-Review: Derek Bender <d24d8a7ee520e34b1cec38800225d9f36b552df7@instructure.com>

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