title: Trouble installing libzmq3-dev on GitHub Actions Ubuntu runners
body: <p>I am having trouble installing <code>libzmq3-dev</code> on Ubuntu 16.04 on GitHub Actions. <strong>This may be a user error on my part because I do not run into an error when I install it on my local Ubuntu machine.</strong> I would appreciate troubleshooting advice, e.g. libraries I should be installing prior to <code>libzmq3-dev</code> on GHA machines specifically.</p>
  <p>I posted a minimal reproducible example to <a href="https://github.com/wlandau/libzmq-reprex/runs/2521101152" rel="nofollow noreferrer">https://github.com/wlandau/libzmq-reprex/runs/2521101152</a>.
  Related: <a href="https://stackoverflow.com/questions/64024478/cannot-install-libzmq3-dev-on-ubuntu-19-04">Cannot install libzmq3-dev on ubuntu 19.04</a>.</p>
  <pre class="lang-sh prettyprint-override"><code>Run sudo apt-get install libzmq3-dev
  Reading package lists...
  Building dependency tree...
  Reading state information...
  Some packages could not be installed. This may mean that you have
  requested an impossible situation or if you are using the unstable
  distribution that some required packages have not yet been created
  or been moved out of Incoming.
  The following information may help to resolve the situation:
  
  The following packages have unmet dependencies:
   libzmq3-dev : Depends: libzmq5 (= 4.1.4-7ubuntu0.1) but 4.3.1-5+ubuntu16.04.1+deb.sury.org+1 is to be installed
  E: Unable to correct problems, you have held broken packages.
  Error: Process completed with exit code 100.
  </code></pre>