ci: Always archive artifacts, only publish master

Sometimes, I want to see what the built site looks like before
publishing it.  For that, I'll push changes to a dev branch and let
Jenkins build the site as a tarball that I can download, extract, and
view locally.  Once I am satisfied, I'll merge the dev branch to master,
which Jenkins will build and publish to the live site.

ci/Jenkinsfile

@@ -26,9 +26,17 @@ pipeline {
                     sh '. ci/build.sh'
                 }
             }
+            post {
+                success {
+                    archiveArtifacts 'dustin.web.tar.xz'
+                }
+            }
         }
 
         stage('Publish') {
+            when {
+                branch 'master'
+            }
             steps {
                 container('rsync') {
                     sshagent(['jenkins-web']) {

ci/build.sh

@@ -5,3 +5,5 @@ from json import dump
 import sys
 dump(load(sys.stdin), sys.stdout)
 EOF
+
+tar -cJf dustin.web.tar.xz -C public .