Sled-agent: detect switch backend (tofino, softnpu, et al) at runtime - #11237

control: job assigned to worker 01M1QHHQ8SHA9B0MN139RCSNN3 [factory aws, i-08b3c50980f3eee8f] (queued for 54 s)
...
task: starting task 0: "setup"
task: process exited: duration 4209 ms, exit code 0
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task: starting task 1: "rust-toolchain"
task: process exited: duration 16325 ms, exit code 0
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task: starting task 2: "authentication"
task: process exited: duration 18 ms, exit code 0
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task: starting task 3: "clone repository"
task: process exited: duration 13544 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 306178K
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| real     1:05.308600855
| user     1:03.181081507
| sys         3.485149773
| trap        0.005329416
| tflt        0.000000000
| dflt        0.000000000
| kflt        0.000000000
| lock        0.000000000
| slp      1:03.127249097
| lat         0.814427842
| stop        0.000175838
task: process exited: duration 1100841 ms, exit code 0
|W| found 1 output files
|W| uploading: /work/package.tar.gz (1836027436 bytes)
|W| uploaded: /work/package.tar.gz