[infra] Move Swiftshader to MEDIUM GCE machines

Change-Id: I9d9c1ee0aacba8598869fb702436a7c6bf668323
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/270838
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner aka dogben <benjaminwagner@google.com>
2 files changed