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Pyodide: fix a temp-file rename race in concurrent asset downloads (#449)

ensureLocalPyodideAssets wrote each cold-cache asset to a shared
"<dest>.tmp" path before renaming it into place. When two callers race a
cold cache concurrently (e.g. several Jest workers, each loading a fresh
pyodide instance in test:pyodide's CI step), both write to the same tmp
path; whichever renames first consumes it, so the other's own rename then
fails with ENOENT ("no such file or directory, rename
'pyodide.asm.js.tmp' -> 'pyodide.asm.js'"), crashing that test file.

Give the tmp path a per-call unique suffix (pid + random) instead, so
concurrent downloads never share one tmp file. Node's fs.rename overwrites
an existing destination atomically rather than erroring, so the loser of
the race just harmlessly overwrites dest with identical content (both
downloaded the same URL) instead of crashing.

Verified locally: yarn test:pyodide passes repeatedly against a cold
/tmp cache (rm -rf'd between runs), where it previously could hit this
race.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

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