wire: wire: test flakes on Travis with "failed to cache compiled Go files"

Describe the bug

The Wire tests fail intermittently on Travis with an error like:

        wire_test.go:96: go [list -e -json -compiled -test=false -export=false -deps=true -tags=wireinject -- example.com/foo]: exit status 1: go: finding github.com/google/go-cloud v0.2.0
            go build runtime/internal/atomic: failed to cache compiled Go files

To Reproduce

This is an intermittent failure, but I’ve only seen it on Travis. Uncertain what the triggers are.

Expected behavior

Wire tests should pass.

/cc @vangent, who I believe hit this failure before, but I can’t find evidence of it.