Add a internal/cmd/update-protos command which regenerates all
protobuf files within the repo, using the versions of the
code generators listed in the go.mod file.
Replace the per-directory //go:generate commands with a single
top-level one.
Regenerate all .pb.go files.
For golang/go#69732
Change-Id: I337badfb49fc29f638d71eee609d5995262ccd75
Reviewed-on: https://go-review.googlesource.com/c/build/+/617057
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Damien Neil