When --experimental-dual-fund is enabled upfront shutdown_scriptpubkey does not follow bolt

[dunxen]

I've been doing some early testing with our in-progress dual-funding implementation in LDK, and it seems that the (mis)behaviour described in #3815 comes up again when --experimental-dual-fund flag is set for Core Lightning and attempting to open an outbound dual-funded channel to LDK.

I'll provide a repro if needed but it would involve an in-progress fork of our ldk-sample node. Logs are the same as #3815, with LDK failing the channel after receiving open_channel2 due to shutdown_scriptpubkey TLV not being set by CLN when both nodes advertise option_upfront_shutdown_script.

Apologies for necromancing!