Add tini as PID 1, guest clock sync, and fix PATH in guest VMs

- Use tini as PID 1 in wrenn-init.sh so zombie processes are reaped
  and signals are forwarded correctly to envd
- Set standard PATH in wrenn-init.sh so child processes spawned by envd
  can find common binaries (fixes "nice: ls command not found")
- Add envdclient.Init() to POST /init on envd after every boot/resume,
  syncing the guest clock via unix.ClockSettime — critical after snapshot
  resume where the guest clock is frozen
- Run Init in a background goroutine so it doesn't block the CreateSandbox
  RPC response; a slow Init (vCPU busy with envd startup) was causing the
  RPC context to be canceled before the response reached the control plane
- Update rootfs-from-container.sh and update-debug-rootfs.sh to inject
  tini into the rootfs, checking the container image and host first,
  downloading from GitHub releases as fallback

images/wrenn-init.sh

@@ -23,5 +23,8 @@ hostname sandbox
 echo "nameserver 8.8.8.8" > /etc/resolv.conf
 echo "nameserver 8.8.4.4" >> /etc/resolv.conf
 
-# Exec envd as the main process (replaces this script, keeps PID 1).
-exec /usr/local/bin/envd
+# Set a standard PATH so envd and all child processes can find common binaries.
+export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+
+# Exec tini as PID 1 — it reaps zombie processes and forwards signals to envd.
+exec /sbin/tini -- /usr/local/bin/envd