Mount /etc/shadow into containers

This fixes `sudo` in Ubuntu 20.04, which printed an error before:

sudo: account validation failure, is your account locked?

This is a safe change, because /etc/shadow does not contain any passwords on our systems.
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 0a5f9df..109334c 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -2068,6 +2068,7 @@
                 "volumes": [
                     "/etc/group:/etc/group:ro",
                     "/etc/passwd:/etc/passwd:ro",
+                    "/etc/shadow:/etc/shadow:ro",
                     "/opt:/opt:ro",
                     "/var/lib/buildkite-agent:/var/lib/buildkite-agent",
                     "/var/lib/gitmirrors:/var/lib/gitmirrors:ro",