[go.mod] Update luci-go pin

The old authentication method used by luci-go no longer works
(see https://crbug.com/1406354) so we must upgrade to the latest version
of luci-go that uses the supported authentication method.

Steps:
1. `go get -u go.chromium.org/luci`
2. `go mod tidy`

Change-Id: I98c7f9430d2c5a9650c30b85ece8311f97f8fd72
Reviewed-on: https://fuchsia-review.googlesource.com/c/tools/+/803609
Reviewed-by: Anthony Fandrianto <atyfto@google.com>
Fuchsia-Auto-Submit: Oliver Newman <olivernewman@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>