Another build break

Nobody has claimed this yet.

Yesterday's fix worked, simple reproduction app now builds, but my larger app fails with a different error:
```
ember-cli:command `build:fallback` command running `Build` task +3ms
- Building broccoli-stew:rename [Funnel] from: .js to: .fb.js +0ms
broccoli-stew:rename [Funnel] from: .html to: .fb.html +2ms
ember-cli:command An error occurred running `Build` from the `build:fallback` command. BuilderError: Directory not found: C:\Users\z\AppData\Local\Temp\tmp-47004Hz2HgWt1lk0L\legacy
at Builder.checkInputPathsExist (P:\Project\node_modules\broccoli\lib\builder.js:268:15)
at new Builder (P:\Project\node_modules\broccoli\lib\builder.js:66:10)
at buildsMerger (P:\Project\node_modules\ember-cli-fallback-builds\lib\utils\builds-merger.js:48:19)
at BuildTask.run (P:\Project\node_modules\ember-cli-fallback-builds\lib\tasks\build.js:85:16)
at Promise.resolve.then (P:\Project\node_modules\ember-cli\lib\models\command.js:239:24)
at tryCatcher (P:\Project\node_modules\ember-cli\node_modules\rsvp\dist\rsvp.js:326:21)
at invokeCallback (P:\Project\node_modules\ember-cli\node_modules\rsvp\dist\rsvp.js:498:33)
at P:\Project\node_modules\ember-cli\node_modules\rsvp\dist\rsvp.js:562:16
at flush (P:\Project\node_modules\ember-cli\node_modules\rsvp\dist\rsvp.js:2441:7)
at process._tickCallback (internal/process/next_tick.js:61:11)
at Function.Module.runMain (internal/modules/cjs/loader.js:757:11)
at startup (internal/bootstrap/node.js:283:19)
at bootstrapNodeJSCore (internal/bootstrap/node.js:622:3)
+15s
ember-cli:project getProjectRoot P:\Project -> P:\Project +3ms
ember-cli:project getProjectRoot P:\Project -> P:\Project +2ms
Directory not found: C:\Users\z\AppData\Local\Temp\tmp-47004Hz2HgWt1lk0L\legacy
```
Any idea what could cause this, or should I debug it more?

Reproduce the larger application's build failure, then start with the stack-trace locations in node_modules/broccoli/lib/builder.js, ember-cli-fallback-builds/lib/utils/builds-merger.js, and ember-cli-fallback-builds/lib/tasks/build.js. Trace why the temporary legacy directory is missing and document or fix the cause, with the larger application building successfully as the completion check.

Written by the indexing model from the issue text.