[nexus] consolidate the VMM unregistration saga helper - #11698

control: job assigned to worker 01M6SP89ER3MDP53140V621785 [factory aws, i-0d67d2eef16110169] (queued for 31 m 46 s)
...
task: starting task 0: "setup"
task: process exited: duration 4073 ms, exit code 0
...
task: starting task 1: "rust-toolchain"
task: process exited: duration 17911 ms, exit code 0
...
task: starting task 2: "authentication"
task: process exited: duration 14310 ms, exit code 0
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task: starting task 3: "clone repository"
task: process exited: duration 19109 ms, exit code 0
...
task: starting task 4: "build"
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|     = help: to override `-D warnings` add `#[allow(unused_variables)]`
| 
| error: could not compile `omicron-nexus` (lib) due to 6 previous errors
| warning: build failed, waiting for other jobs to finish...
| error: could not compile `omicron-nexus` (lib test) due to 6 previous errors
| Error: failed: exit status: 101
| 
| real     3:45.579973527
| user    19:29.600482423
| sys      4:45.404891491
| trap        0.497368357
| tflt        0.922271050
| dflt        1.023687806
| kflt        0.038860537
| lock    37:30.143105055
| slp     53:09.042976116
| lat      1:18.392478899
| stop    14:13.944810317
task: process exited: duration 375284 ms, exit code 1
|W| found 0 output files