fix(code.angularjs.org): correctly re-construct paths on Windows in `sendStoredFile()`

Previously, the `sendStoredFile()` Firebase function used `.split('/')`
to split the request path into segments and later used `path.join()` to
join them back together. This worked fine on *nix based systems, which
use `/` as the path separator. On Windows, however, where `\` is the
path separator, the re-constructed paths could not be retrieved from the
Google Cloud Storage bucket. This prevented the Firebase emulators from
working correctly when testing the function locally on Windows.

This commit fixes the issue by using `.join('/')` to join the path
segments back together.

Closes #17114