fix: fall back to tar install without hanging when package-manager install fails

[zkochan]

Problem

When `--method=package-manager` installation failed (e.g. `ERR_PNPM_TARBALL_INTEGRITY` in CI), the fallback to the tar method ran and succeeded — but the process never exited, so CI jobs hung until killed by a 10-minute no-output timeout, making the whole install look like a failure.

Two separate leaks kept the event loop alive:

1. On the pnpm failure path, `installWithPnpm` never called `global'finishWorkers'`, leaving pnpm's worker threads running.
2. `fetchNode` (`@pnpm/node.fetcher`) also runs actions in pnpm workers, and nothing on the tar path terminated them — so even a successful tar install that downloaded Node.js left the process hanging (reproduced with `--method tar` on an unmodified flow).

There was also no upper bound on how long the package-manager method could take: if pnpm hung instead of throwing, the tar fallback was never reached.

Changes

• `install-with-pnpm.ts`: wrap `install.handler` in `try/finally` so `finishWorkers()` and `stopReporting()` run whether the installation succeeds, fails, or is aborted.
• `install.ts`: terminate pnpm workers in a `finally` when `installVersion` completes, regardless of installation method (covers the `fetchNode` workers).
• `install.ts`: race the package-manager installation against a 60-second timeout. On timeout, an `AbortSignal` is triggered, pnpm's workers are terminated, and installation falls back to the tar method.
• The abort signal is checked before `renameOverwrite`, so a slow registry install that finishes after the timeout can never overwrite the version directory installed by the tar fallback.
• Print the reason when falling back to the tar method instead of failing silently.

Verification (run from the compiled bundle with an isolated `BVM_GLOBALS_DIR`)

Scenario	Before	After
pnpm fails fast (bad lockfile path)	fallback ran, process hung	fallback + install in 43s, clean exit
pnpm hangs (registry → dead port)	no fallback at all	abort at 60s, tar install done at ~1m30s, clean exit
`--method tar` with Node.js download	install done in 28s, process hung forever	clean exit in 29s

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