cmd/containerboot: make the boot map-response timeout configurable

containerboot waits up to 60s for the initial map response before
failing. Slow map responses (>60s seen in production) leaves
containerboot to timeout and fail.

Add a TS_BOOT_TIMEOUT env var to override the default. Falls back to
60s when unset.

Fixes #20912

Signed-off-by: chaosinthecrd <tom@tmlabs.co.uk>