Install reprepro in apt_repo.sh

Kokoro has switched the default Ubuntu image over to 16.04 LTS, which
no longer installs reprepro by default.

BUG=chromium:1174979
TEST=kokoro presubmits

Disallow-Recycled-Builds: test-failures
Change-Id: I56b0593c17a4baf84fc77e8b0591e8a112a3c8e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/containers/cros-container-guest-tools/+/2677862
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Fergus Dall <sidereal@google.com>
diff --git a/kokoro/apt_repo.sh b/kokoro/apt_repo.sh
index 5705fa6..b0b4add 100755
--- a/kokoro/apt_repo.sh
+++ b/kokoro/apt_repo.sh
@@ -10,6 +10,9 @@
 main() {
     require_kokoro_artifacts
 
+    sudo apt-get -q update
+    sudo apt-get -q -y install reprepro
+
     local src_root="${KOKORO_ARTIFACTS_DIR}/git/cros-container-guest-tools"
     local repo_dir="${src_root}"/apt_unsigned
     mkdir -p "${repo_dir}"/{,conf}