build/ci: add "go mod tidy" step

This ensures that go.{mod,sum} are up to date and that there
aren't any missing placeholders.

Change-Id: I2ebc60a8f43f62bf8b70df66e68fafd7026b6148
Reviewed-on: https://review.monogon.dev/c/monogon/+/1107
Reviewed-by: Mateusz Zalega <mateusz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/build/ci/jenkins-presubmit.groovy b/build/ci/jenkins-presubmit.groovy
index cf7b705..7be25c2 100644
--- a/build/ci/jenkins-presubmit.groovy
+++ b/build/ci/jenkins-presubmit.groovy
@@ -52,6 +52,7 @@
                         sh gazelle_build
                         sh "JENKINS_NODE_COOKIE=dontKillMe ~/bazelisk run //:gazelle-update-repos"
                         sh "JENKINS_NODE_COOKIE=dontKillMe ~/bazelisk run //:gazelle -- update"
+                        sh "JENKINS_NODE_COOKIE=dontKillMe ~/bazelisk run //:go -- mod tidy"
 
                         script {
                             def diff = sh script: "git status --porcelain", returnStdout: true