Avoid TOCTOU race when serving console assets - #11090

control: job assigned to worker 01M08EF177FSDCM8BN7ZRE1990 [factory aws, i-0ab5cc7e3730e20da] (queued for 53 s)
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task: starting task 0: "setup"
task: process exited: duration 3139 ms, exit code 0
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task: starting task 1: "rust-toolchain"
task: process exited: duration 17619 ms, exit code 0
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task: starting task 2: "authentication"
task: process exited: duration 24 ms, exit code 0
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task: starting task 3: "clone repository"
task: process exited: duration 11952 ms, exit code 0
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task: starting task 4: "build"
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| + 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
| 136104   32 drwxr-xr-x   2 build    build           2 Aug 17 18:40 /var/tmp/omicron_tmp
| + rmdir /var/tmp/omicron_tmp
task: process exited: duration 1941547 ms, exit code 0
|W| found 6 output files
|W| uploading: /work/cargo-build-analysis.jsonl (1798233 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 (657916 bytes)
|W| uploaded: /work/oxidecomputer/omicron/target/nextest/ci/junit.xml
|W| uploading: /work/oxidecomputer/omicron/target/live-tests-archive.tgz (281128448 bytes)
|W| uploaded: /work/oxidecomputer/omicron/target/live-tests-archive.tgz
|W| uploading: /tmp/nextest-run-archive.zip (3179832 bytes)
|W| uploaded: /tmp/nextest-run-archive.zip
|W| uploading: /tmp/nextest-chrome-trace.json (4190210 bytes)
|W| uploaded: /tmp/nextest-chrome-trace.json