[infra] Update go to 1.15

Change-Id: Id934558f413f0931282931764bd93d3c22c38115
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/311101
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
Auto-Submit: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
5 files changed