[infra] Update to Go 1.12.4

Change-Id: Icbb1adbb7276ef4e6952577db34bbc55b72ea4e6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/211422
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
3 files changed