Avoid TOCTOU race when serving console assets - #11090

control: job assigned to worker 01M08EF4QX3TYTH8MRPPFE996P [factory aws, i-0d0a9185278116121] (queued for 51 s)
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task: starting task 0: "setup"
task: process exited: duration 4098 ms, exit code 0
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task: starting task 1: "rust-toolchain"
task: process exited: duration 16626 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 13150 ms, exit code 0
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task: starting task 4: "build"
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| info: running `cargo check --bins` on zone-setup (334/334)
| warning: profile package spec `blake2b_simd` in profile `dev` did not match any packages
| warning: profile package spec `rust-argon2` in profile `dev` did not match any packages
|     Checking uzers v0.12.2
|     Checking zone-setup v0.1.0 (/work/oxidecomputer/omicron/zone-setup)
|     Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.71s
| 
| real    14:01.777678317
| user    21:05.522501174
| sys      4:21.487323197
| trap        0.323305563
| tflt        0.568568466
| dflt        0.812123888
| kflt        0.004639591
| lock  1:26:37.269194999
| slp   1:40:46.163869476
| lat        57.717572451
| stop     4:02.236520213
task: process exited: duration 973477 ms, exit code 0
|W| found 0 output files