systemctl: gracefully adjust bus transport and runtime scope when --boot-loader-entry...

systemctl: gracefully adjust bus transport and runtime scope when --boot-loader-entry=help

This fixes the following assertion:
===
SYSTEMD_LOG_LEVEL=debug systemctl --user -H foo --boot-loader-entry=help
Assertion 'transport != BUS_TRANSPORT_REMOTE || runtime_scope == RUNTIME_SCOPE_SYSTEM' failed at src/shared/bus-util.c:284, function bus_connect_transport(). Ignoring.
Failed to connect to bus: Operation not supported
===

Fixes a bug introduced by 97af80c5a7029c3f92e982dcf9338b9e67ad9cde.
Fixes #33661.
Fixes oss-fuzz#70153.