title: `conda update --all` without changing package's channel
body: <p>If I have an env as follow,</p>
  
  <pre class="lang-sh prettyprint-override"><code># packages in environment at ~/anaconda3/envs/project3:
  #
  # Name           Version          Build  Channel
  pkg1             1.1              main   anaconda/pkgs/main
  pkg2             2.2              main   conda-forge
  ...              ...              ...    ...
  </code></pre>
  
  <p>The command <code>conda update --all</code> gives me</p>
  
  <pre><code>The following packages will be SUPERSEDED by a higher-priority channel:
  
    pkg2                          conda-forge --&gt; anaconda/pkgs/main
  </code></pre>
  
  <p>The command <code>conda update --all -c conda-forge</code> behaves similar but for <code>pkg1</code>.</p>
  
  <p>How to update all packages with channels remain respectively.</p>