hack: use tools build constraint for generated-files deps

[crazy-max]

fixes #3187
follow-up docker/buildx#1296 (comment)

Inferred go install from go.mod using tools build constraint so proto generation is aligned.

github.com/golang/protobuf/protoc-gen-go switches from 1.3.5 to 1.5.2.

Looking at the release notes and since 1.4.0:

This release of the github.com/golang/protobuf module introduces a number of significant changes relative to the previous minor release. In particular, this module is now implemented in terms of the new google.golang.org/protobuf module, which is the next major revision of Go bindings for protocol buffers. From this point onwards, most of the development effort for Go protobufs will be dedicated to the new module, with minimal changes being made to this module.

The API of this new module is not drop-in compatible with the earlier github.com/golang/protobuf module. See https://github.com/protocolbuffers/protobuf-go/releases/tag/v1.20.0 for backward compat.

[jedevc]

Looks mostly good to me - I have some confusion around pb_tools.go, but otherwise looks good to me.

Nice to see the collection of shell scripts shrinking and moving into Dockerfiles.

[jedevc]

The need for tools.go is still kind of frustrating, but it feels a bit like a necessary evil - it's still an improvement on what we have today, so seems fine to me 🤷

PTAL @tonistiigi

[crazy-max]

As discussed with tonis, I have updated the comment in the package to warn users if they want to add other tools.