fix(security): close the two actionable code-scanning alerts

CodeQL #101 (high, js/insecure-temporary-file) — the SF Symbols.app
install manifest staged at `<cacheRoot>/.manifest.<pid>.<now>.tmp`. The
`wx` flag closed the write race but not the *prediction*: the path is
derivable, so anyone able to write into cacheRoot can pre-place a symlink
and have the manifest land elsewhere. Stage inside a `mkdtemp` directory
instead — kernel-random name, mode 0700 — so neither the file nor its
parent is guessable, matching the staging pattern already used for the
Swift workers. Cleanup moved to `finally` so a failed rename can't leak
the staging dir.

CodeQL #98 (note, js/unused-local-variable) — `tmpdir` was imported in
scripts/_verify-setups.mjs but only ever named in a comment. Dropped.
Neither knip nor biome caught it: knip works at file/export/dependency
granularity and biome runs at --diagnostic-level=error.