Bump timeout to 20 minutes.

Apparently repo will sometimes run slow at checking out files locally on disk,
depending on GKE I/O performance.
diff --git a/vars/functions.groovy b/vars/functions.groovy
index 7f2cd26..a403a54 100644
--- a/vars/functions.groovy
+++ b/vars/functions.groovy
@@ -11,7 +11,7 @@
        chmod 777 cache
        """
 
-    timeout(time: 10, unit: 'MINUTES') {
+    timeout(time: 20, unit: 'MINUTES') {
         checkout([$class: 'RepoScm',
                   currentBranch: true,
                   jobs: 24,