fix: ensure we respect DOKKU_DEPLOY_BRANCH when rebuilding applications

Closes #3021

tests/unit/20_config.bats

@@ -153,9 +153,19 @@ teardown() {
   echo "status: "$status
   assert_success
 
+  run ssh dokku@dokku.me ps:rebuild $TEST_APP
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
   run ssh dokku@dokku.me config:set $TEST_APP DOKKU_DEPLOY_BRANCH=app-branch
   GIT_REMOTE_BRANCH=app-branch deploy_app
   echo "output: "$output
   echo "status: "$status
   assert_success
+
+  run ssh dokku@dokku.me ps:rebuild $TEST_APP
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
 }