[infra] Upgrade Debian GCE bots to 9.8.

Change-Id: I4996779ad7a634bce6e39eb6defa4bc0cea026d7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/197203
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
2 files changed