git: Setup git-cookie-authdaemon for oauth service account

This should allow the gcompute default service account access to internal
repositories.
diff --git a/vars/functions.groovy b/vars/functions.groovy
index a403a54..726af49 100644
--- a/vars/functions.groovy
+++ b/vars/functions.groovy
@@ -6,6 +6,10 @@
        apt-get -y install make python curl git qemu-user-static sudo
        curl https://storage.googleapis.com/git-repo-downloads/repo > /usr/local/bin/repo
        chmod a+x /usr/local/bin/repo
+
+       git clone https://gerrit.googlesource.com/gcompute-tools /tmp/gcompute-tools
+       /tmp/gcompute-tools/git-cookie-authdaemon
+
        mkdir -p .repo/local_manifests
        mkdir -p cache
        chmod 777 cache