[email protected] calls find-cache-dir({ name: 'gh-pages' }) for both
clean() and publish(). find-cache-dir walks up from cwd looking for a
package.json and returns undefined when it finds none. gh-pages then
does path.join(undefined, ...) and throws:

  The "path" argument must be of type string or an instance of Buffer
  or URL. Received undefined

This hit users running `npx angular-cli-ghpages` from a repo that is
just a dist/ folder (no package.json). The reporter's workaround of
`npm install angular-cli-ghpages` first worked because it created a
package.json that find-cache-dir could find.

Fix: add ensureGhPagesCacheDir() to engine.prepare-options-helpers and
call it from engine.run() before gh-pages is required. If no
package.json is reachable from cwd, we set process.env.CACHE_DIR to
`<os.tmpdir()>/angular-cli-ghpages-cache`. find-cache-dir honors
CACHE_DIR as an explicit override — when set (and not a boolean-ish
value 'true'/'false'/'1'/'0') it short-circuits its package.json walk.

We do the package.json probe ourselves rather than delegating to
find-cache-dir: find-cache-dir captures `const {env} = process` at
module load and is then cached in require.cache, which makes it
brittle under test reassignment of process.env.

Tests:

- engine.prepare-options-helpers.spec.ts — unit tests covering: cwd
  with no package.json sets fallback; cwd with package.json is a
  no-op; pre-existing user-set CACHE_DIR is preserved; boolean-ish
  CACHE_DIR values ('true'/'false'/etc.) are treated as unset per
  find-cache-dir's own semantics; fallback path is deterministically
  inside os.tmpdir().

- engine.gh-pages-no-package-json.spec.ts — real-git, real-gh-pages
  end-to-end. Creates a bare repo + workdir with no package.json in
  any ancestor, process.chdir()s there, runs engine.run(), inspects
  the remote tree, asserts the dist actually lands. A companion
  baseline test spawns a child Node process (no shared require.cache)
  and calls bare gh-pages.clean() in the same workdir, asserting it
  throws the original TypeError — proving the upstream bug is real.

Fixes #203