proto-gae: output build tag in new format introduced in go 1.17 as well
proto-gae: output build tag in new format introduced in go 1.17 as well

See: https://pkg.go.dev/cmd/go#hdr-Build_constraints

R=gregorynisbet, iannucci, vadimsh

Change-Id: I323d216385daf8ea449559c3eb5e3542b4fd16bc
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3907693
Commit-Queue: Gregory Nisbet <afc3bca0abcb2567b0a2495a821fca959b52ca27@google.com>
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Gregory Nisbet <afc3bca0abcb2567b0a2495a821fca959b52ca27@google.com>
Auto-Submit: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

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