[installinator] write out mupdate-override.json

[sunshowers]

Write out a file containing a UUID which indicates that installinator wrote out an updated dataset. This is a signal for Nexus to assume that installinator wrote out a new install dataset behind its back.

I'm not entirely sure if writing out the JSON file at the beginning vs at some later point matters a huge amount here -- if installinator fails for some reason, we assume the system is broken anyway.

We also record the host phase 2 and control plane artifact hashes, and there's space to record other hashes if we'd like to do so in the future.

Tested on dublin:

BRM42220026 # ls -al /pool/int/*/install/mupdate-override.json
-rw-------   1 root     root          55 Dec 28  1986 /pool/int/2196ba5d-e211-4495-9c0b-e0948ba0afbd/install/mupdate-override.json
-rw-------   1 root     root          55 Dec 28  1986 /pool/int/f3673b2a-0cdc-4315-ac53-7889918cee4c/install/mupdate-override.json

See RFD 556 for more information.