[CT] Install python3.8
[CT] Install python3.8

Bug: skia:12021
Change-Id: I0c61332a1055bb816d02fe7f95c8c8e04c241f1b
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/410323
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot