.gitignore: add go.mod/go.sum
.gitignore: add go.mod/go.sum

context:
https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1792504/6#message-986f872f615033106c6b98de1c9b1e75ec9c9a06

Bug: 1057067
Change-Id: I6df62fa242f98162cc579c8e8152beed4d3bd9bb
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2162431
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go