title: How to use go list -u -m all with vendoring mode?
body: <p>I just upgraded to go1.14 and running <code>go list -u -m all</code> command with -mod=vendor GOFLAG , i encountered <strong>can't determine available upgrades using the vendor directory</strong>. I went through the release notes (<a href="https://golang.org/doc/go1.14#go-command" rel="nofollow noreferrer">https://golang.org/doc/go1.14#go-command</a>) but couldn't figure out why this error. ( My go.mod and vendor/modules.txt were in sync ).</p>
  <p>When to use <code>-mod=readonly</code> like <code>go list -mod=readonly -u -m all</code> command? What's wrong with using  <code>go list -u -m all</code> with <code>-mod=vendor</code>?</p>
  <p>Any pointers would be much appreciated . Thank you.</p>
  <pre><code>GOGOPATH=/home/vagrant/go_workspace
  GO111MODULE=on
  GOFLAGS=-mod=vendor
  ============================================
  go list  -u -m all
  go list -m: can't determine available upgrades using the vendor directory
          (Use -mod=mod or -mod=readonly to bypass.)
  
  </code></pre>