Relax an eager sled agent assert - #10401

control: job assigned to worker 01M5DJBESF71ZA0AFHC17ZPTYB [factory aws, i-06da7b6d150f850e6] (queued for 50 s)
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task: starting task 0: "setup"
task: process exited: duration 4101 ms, exit code 0
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task: starting task 1: "rust-toolchain"
task: process exited: duration 17037 ms, exit code 0
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task: starting task 2: "authentication"
task: process exited: duration 22 ms, exit code 0
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task: starting task 3: "clone repository"
task: process exited: duration 19189 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 37167K
| a out/pumpkind-gz.tar 26647K
| a out/switch-softnpu.tar.gz 331818K
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| real     1:07.359596944
| user     1:05.156003973
| sys         3.627804298
| trap        0.005165881
| tflt        0.000000000
| dflt        0.000000000
| kflt        0.000000000
| lock        0.000000000
| slp      1:05.105613624
| lat         0.819858550
| stop        0.000210490
task: process exited: duration 1007604 ms, exit code 0
|W| found 1 output files
|W| uploading: /work/package.tar.gz (1882993517 bytes)
|W| uploaded: /work/package.tar.gz