Use the correct path when checking if a `/l10n` directory exists, to avoid errors during `gulp importl10n` (PR 7063 followup)
Use the correct path when checking if a `/l10n` directory exists, to avoid errors during `gulp importl10n` (PR 7063 followup)

This is regression from PR 7063, causing `gulp importl10n` to fail:
```
$ gulp importl10n
[19:45:02] Using gulpfile c:\Users\Jonas\Git\pdfjs\gulpfile.js
[19:45:02] Starting 'importl10n'...

Downloading ach...
[19:45:02] 'importl10n' errored after 4.42 ms
[19:45:02] Error: EEXIST, file already exists 'c:\Users\Jonas\Git\pdfjs\l10n\ach
'
    at Error (native)
    at Object.fs.mkdirSync (fs.js:747:18)
    at downloadLanguageFiles (c:\Users\Jonas\Git\pdfjs\external\importL10n\local
es.js:59:8)
    at next (c:\Users\Jonas\Git\pdfjs\external\importL10n\locales.js:90:5)
    at Object.downloadL10n (c:\Users\Jonas\Git\pdfjs\external\importL10n\locales
.js:91:5)
    at Gulp.<anonymous> (c:\Users\Jonas\Git\pdfjs\gulpfile.js:92:11)
    at module.exports (c:\Users\Jonas\Git\pdfjs\node_modules\gulp\node_modules\o
rchestrator\lib\runTask.js:34:7)
    at Gulp.Orchestrator._runTask (c:\Users\Jonas\Git\pdfjs\node_modules\gulp\no
de_modules\orchestrator\index.js:273:3)
    at Gulp.Orchestrator._runStep (c:\Users\Jonas\Git\pdfjs\node_modules\gulp\no
de_modules\orchestrator\index.js:214:10)
    at Gulp.Orchestrator.start (c:\Users\Jonas\Git\pdfjs\node_modules\gulp\node_
modules\orchestrator\index.js:134:8)
```

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