title: Joining environment variables in Jenkins pipeline / groovy
body: <p>I'm trying to set a couple of environment variables in a Jenkinsfile, but my lack of Java/Groovy-ness seems to be failing me...</p>
  
  <pre><code>pipeline {
      agent any
      environment {
          TMPDIR = /mnt/storage/build
          TOX_DIR = $TMPDIR/$BUILD_TAG
      }
  ...
  </code></pre>
  
  <p>Generates the following error on the console...</p>
  
  <pre><code>WorkflowScript: 7: Environment variable values can only be joined together with +s. @ line 7, column 26.
         TOX_DIR = $TMPDIR/$BUILD_TAG
  </code></pre>
  
  <p>Other variations such as ...</p>
  
  <pre><code>TOX_DIR = "$TMPDIR" + "/" + "$BUILD_TAG"
  </code></pre>
  
  <p>or</p>
  
  <pre><code>TOX_DIR = "$TMPDIR/$BUILD_TAG"
  </code></pre>
  
  <p>or</p>
  
  <pre><code>TOX_DIR = "${TMPDIR}/${BUILD_TAG}"
  </code></pre>
  
  <p>only make matters worse.</p>
  
  <p>What am I missng?     Thanks....</p>