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

control: job assigned to worker 01M6SP8868XVQY0X4EGGYWWW4D [factory aws, i-0de204188df78213e] (queued for 31 m 43 s)
...
task: starting task 0: "setup"
task: process exited: duration 4096 ms, exit code 0
...
task: starting task 1: "rust-toolchain"
task: process exited: duration 19250 ms, exit code 0
...
task: starting task 2: "authentication"
task: process exited: duration 183 ms, exit code 0
...
task: starting task 3: "clone repository"
task: process exited: duration 11266 ms, exit code 0
...
task: starting task 4: "build"
...
| a out/probe.tar.gz 3037K
| a out/propolis-server.tar.gz 37167K
| a out/pumpkind-gz.tar 26647K
| a out/switch-softnpu.tar.gz 334567K
| 
| real     1:06.694815469
| user     1:04.535976692
| sys         3.592479309
| trap        0.003895094
| tflt        0.000000000
| dflt        0.000000000
| kflt        0.000000000
| lock        0.000000000
| slp      1:04.452963757
| lat         0.800487373
| stop        0.000155540
task: process exited: duration 1073946 ms, exit code 0
|W| found 1 output files
|W| uploading: /work/package.tar.gz (1878770530 bytes)
|W| uploaded: /work/package.tar.gz