roll_dep.py: Fix outstanding references to subprocess
roll_dep.py: Fix outstanding references to subprocess

https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2512921
changed roll_dep.py to use subprocess2 instead of subprocess but some
references weren't updated.

Bug: chromium:1156908
Change-Id: Icac020b463d1b68a4fca5b4ddf5cb80b9bfa9516
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2581545
Commit-Queue: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>
Reviewed-by: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>
Auto-Submit: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools