The Go project has all build failed logs storage at GCE, which you can access by the dashboard.
You can download all the failed logs by fetchlogs.
Download fetchlogs by go get golang.org/x/build/cmd/fetchlogs
fetchlogs only download 300 latest failed logs by default, which is not enough for “mystery”/“flaky” bugs. fetchlogs -n <the number you want>
You may also want to take a look at github.com/aclements/go-misc/greplogs. It's a useful tool by Austin to run grep over logs fetched by fetchlogs.
Also github.com/aclements/findflakes can make some guesses as to when a flaky test started failing.