add README for reconfigurator-exec-unsafe - #11243

control: job assigned to worker 01M1Q9HB7JN310GBMW3FYZMS07 [factory aws, i-0def9651af63c1216] (queued for 51 s)
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task: starting task 0: "setup"
task: process exited: duration 4146 ms, exit code 0
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task: starting task 1: "rust-toolchain"
task: process exited: duration 15637 ms, exit code 0
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task: starting task 2: "authentication"
task: process exited: duration 17 ms, exit code 0
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task: starting task 3: "clone repository"
task: process exited: duration 14646 ms, exit code 0
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task: starting task 4: "build"
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| a out/probe.tar.gz 3037K
| a out/propolis-server.tar.gz 34642K
| a out/pumpkind-gz.tar 26647K
| a out/switch-softnpu.tar.gz 306132K
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| real     1:05.441507455
| user     1:03.293209356
| sys         3.520142271
| trap        0.005034107
| tflt        0.000000000
| dflt        0.000000000
| kflt        0.000000000
| lock        0.000000000
| slp      1:03.244242380
| lat         0.815974056
| stop        0.000142780
task: process exited: duration 1044188 ms, exit code 0
|W| found 1 output files
|W| uploading: /work/package.tar.gz (1837023738 bytes)
|W| uploaded: /work/package.tar.gz