add README for reconfigurator-exec-unsafe - #11243

control: job assigned to worker 01M1Q9H6X4PAYGZ0NSENNGTQBW [factory aws, i-0a51f19e5535ee733] (queued for 28 s)
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task: starting task 0: "setup"
task: process exited: duration 18317 ms, exit code 0
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task: starting task 1: "rust-toolchain"
task: process exited: duration 8254 ms, exit code 0
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task: starting task 2: "authentication"
task: process exited: duration 5 ms, exit code 0
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task: starting task 3: "clone repository"
task: process exited: duration 8504 ms, exit code 0
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task: starting task 4: "build"
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| + [[ linux == \i\l\l\u\m\o\s ]]
| + rm -rf /var/tmp/omicron_tmp/crdb-base-build
| + unset TMPDIR
| + echo 'files in /var/tmp/omicron_tmp (none expected on success):'
| files in /var/tmp/omicron_tmp (none expected on success):
| + find /var/tmp/omicron_tmp -ls
|    258225     20 drwxr-xr-x   2 build    build       20480 Sep  4 23:14 /var/tmp/omicron_t [...]
| + rmdir /var/tmp/omicron_tmp
task: process exited: duration 1712067 ms, exit code 0
|W| found 5 output files
|W| uploading: /work/cargo-build-analysis.jsonl (1787565 bytes)
|W| uploaded: /work/cargo-build-analysis.jsonl
|W| uploading: /work/environment.json (108 bytes)
|W| uploaded: /work/environment.json
|W| uploading: /work/oxidecomputer/omicron/target/nextest/ci/junit.xml (659650 bytes)
|W| uploaded: /work/oxidecomputer/omicron/target/nextest/ci/junit.xml
|W| uploading: /tmp/nextest-run-archive.zip (3121086 bytes)
|W| uploaded: /tmp/nextest-run-archive.zip
|W| uploading: /tmp/nextest-chrome-trace.json (4142861 bytes)
|W| uploaded: /tmp/nextest-chrome-trace.json