Add apt-wait.sh, which waits for an existing apt process to finish (#83)

Sometimes, our CI job which installs dependencies using `apt` would fail because the machine may perform an initial `apt` update in the background. This PR adds `apt-wait.sh` for waiting for an existing `apt` to finish.

Author: Ganeshwara Herawan Hananda

ci/BUILD

@@ -75,4 +75,4 @@ py_binary(
     name = "test-cache",
     srcs = ["test-cache.py"],
     main = "test-cache.py"
-)
+)

ci/apt-wait.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+echo -n 'Waiting for the initial apt-get process to finish...'
+init_wait=0
+while [[ $init_wait -eq 0 ]]; do
+  set +e
+  ps -C apt-get > /dev/null
+  init_wait=$?
+  set -e
+  echo -n '.'
+  sleep 1
+done
+echo 'done.'