Avoid TOCTOU race when serving console assets - #11090

control: job assigned to worker 01M08EF86SSY6EQH50FX1YVHCP [factory aws, i-0c12310d24e07b621] (queued for 53 s)
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task: starting task 0: "setup"
task: process exited: duration 4086 ms, exit code 0
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task: starting task 1: "rust-toolchain"
task: process exited: duration 17320 ms, exit code 0
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task: starting task 2: "authentication"
task: process exited: duration 16 ms, exit code 0
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task: starting task 3: "clone repository"
task: process exited: duration 18849 ms, exit code 0
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task: starting task 4: "build"
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|  Documenting typed-rng v0.1.0 (/work/oxidecomputer/omicron/typed-rng)
|  Documenting omicron-workspace-hack v0.1.0 (/work/oxidecomputer/omicron/workspace-hack)
|  Documenting omicron-uuid-kinds v0.1.0 (/work/oxidecomputer/omicron/uuid-kinds)
|  Documenting dev-tools-common v0.1.0 (/work/oxidecomputer/omicron/dev-tools/common)
|     Finished `dev` profile [unoptimized + debuginfo] target(s) in 7m 12s
|    Generated /work/oxidecomputer/omicron/target/doc/api_identity/index.html and 202 other  [...]
| 
| real     7:12.969143304
| user    19:53.631970921
| sys      4:18.808694667
| trap        0.296958932
| tflt        0.005794840
| dflt        0.053034210
| kflt        0.000012438
| lock  6:49:50.368542539
| slp   2:24:53.322088229
| lat        38.752600548
| stop     2:00.434907792
task: process exited: duration 816351 ms, exit code 0
|W| found 0 output files