Fetch latest tags before testing whether current build is a tag

The two most recent client releases failed to publish automatically
because the latest tags had not been fetched prior to running
`git describe` to check whether the git ref was a tag.

I'm not sure exactly what changed and why this used to work before,
but add an explicit `git fetch --tags` step.

eg. See https://jenkins.hypothes.is/job/client/job/master/483/

Jenkinsfile

@@ -41,6 +41,7 @@ node {
 
 boolean isTag() {
     try {
+        sh 'git fetch --tags'
         sh 'git describe --exact-match --tags'
         return true
     } catch (Exception e) {