title: covr::package_coverage reports "No such file or directory"
body: <p>I'm trying to see the code coverage of <a href="https://github.com/sumbose/iRF" rel="noreferrer">sumbose/iRF</a>, so I did a <code>git clone</code>, started an R session inside of the directory, and</p>
  
  <pre><code>&gt; library(covr)
  &gt; package_coverage()
  Error in file(con, "r") : cannot open the connection
  In addition: Warning messages:
  1: In utils::install.packages(repos = NULL, lib = tmp_lib, pkg$path,  :
    installation of package ‘/private/tmp/iRF’ had non-zero exit status
  2: In file(con, "r") :
    cannot open file '/private/var/folders/ny/f06ns0d568bgf6s559z8j_9m0000gn/T/RtmpAr8dLV/R_LIBS168866d1ef32f/iRF/R/iRF': No such file or directory
  </code></pre>
  
  <p>However, both <code>R CMD INSTALL iRF</code> and <code>install.packages('iRF', repos = NULL)</code> installs the package as expected.</p>