fix(npmRegistryUtils): point users at the real env vars when integrity check fails

[spokodev]

The signature-failure path in `fetchLatestStableVersion` tells users they can disable the integrity check with `COREPACK_INTEGRITY_CHECK=0` and fall back to the bundled latest release with `COREPACK_USE_LATEST=0`. Neither variable exists anywhere else in the codebase. The runtime reads `COREPACK_INTEGRITY_KEYS` (via `shouldSkipIntegrityCheck` in `sources/corepackUtils.ts`) and `COREPACK_DEFAULT_TO_LATEST` (in `sources/corepackUtils.ts` and `sources/Engine.ts`), and those are the names the README advertises.

A user who hits the error and follows the in-message suggestion sets two variables that do nothing and keeps hitting the same wall.

```shell
$ rg COREPACK_INTEGRITY_CHECK sources/ README.md
sources/npmRegistryUtils.ts:83:...COREPACK_INTEGRITY_CHECK to 0 in your env...

$ rg COREPACK_USE_LATEST sources/ README.md
sources/npmRegistryUtils.ts:83:...COREPACK_USE_LATEST to 0

$ rg COREPACK_INTEGRITY_KEYS sources/ README.md
sources/corepackUtils.ts:451: return process.env.COREPACK_INTEGRITY_KEYS === ``
sources/corepackUtils.ts:452: || process.env.COREPACK_INTEGRITY_KEYS === `0`;
sources/npmRegistryUtils.ts:43: const {npm: trustedKeys} = process.env.COREPACK_INTEGRITY_KEYS ?
sources/npmRegistryUtils.ts:44: JSON.parse(process.env.COREPACK_INTEGRITY_KEYS) as ...
README.md:...

$ rg COREPACK_DEFAULT_TO_LATEST sources/ README.md
sources/Engine.ts:182: if (process.env.COREPACK_DEFAULT_TO_LATEST === `0`) {
sources/corepackUtils.ts:336: if (... process.env.COREPACK_DEFAULT_TO_LATEST !== `0`) {
README.md:...
```

Change

`sources/npmRegistryUtils.ts:83` - reworded the thrown error to point at `COREPACK_INTEGRITY_KEYS` and `COREPACK_DEFAULT_TO_LATEST`. The behaviour of `fetchLatestStableVersion` is otherwise unchanged.

Tests

Two cases in `tests/npmRegistryUtils.test.ts` under a new `fetchLatestStableVersion` describe block:

• positive + negative axis in one test: when verification fails the thrown error contains the real names (`COREPACK_INTEGRITY_KEYS to 0`, `COREPACK_DEFAULT_TO_LATEST to 0`) and does not contain the phantom names. Fails on `main` (the assertion for the new strings does not match the current message); passes on the branch.
• regression guard: when `COREPACK_INTEGRITY_KEYS=0` is set, `fetchLatestStableVersion` skips verification and returns the resolved version, so the env var name we now advertise is the one the runtime actually obeys.

Verification

`corepack yarn vitest run tests/npmRegistryUtils.test.ts` -> 9 passed (7 existing + 2 new).
`corepack yarn lint` clean on the changed files.
`corepack yarn typecheck` clean.

Fixes #849