CI: make dev.setup fails too often due to Go proxy stream errors

[superleomori]

Problem

make dev.setup (step 1037 in CI) fails intermittently with a Go module proxy stream error:

go: github.com/google/go-github/v84@v84.0.0: read "https://proxy.golang.org/github.com/google/go-github/v84/@v/v84.0.0.zip": stream error: stream ID 2211; INTERNAL_ERROR; received from peer
make[1]: *** [Makefile:118: dev.setup.go] Error 1
make: *** [Makefile:108: dev.setup] Error 2

This is a transient network/proxy error from proxy.golang.org that kills the entire setup step (~41 seconds in).

Impact

• Flaky CI — make dev.setup fails for reasons unrelated to code changes
• Wastes CI time and blocks PRs

Fix

• Add retry logic around go mod download / go get in the Makefile or CI config
• Consider vendoring dependencies (go mod vendor) to eliminate proxy dependency entirely
• Or use GONOSUMCHECK / GOFLAGS=-mod=mod with a retry wrapper