envbuilder: use container entrypoint if ENVBUILDER_INIT_SCRIPT is not set

[johnstcn]

By default, envbuilder runs /bin/sh -c sleep infinity as the entrypoint. This behaviour can be surprising, so we should default to the entrypoint defined in the container if ENVBUILDER_INIT_SCRIPT / ENVBUILDER_INIT_COMMAND are not set.

[Jphalan]

@mafredri Can this be closed?