Fix index generation not wrapping list in firmwares

[TheJulianJES]

It has been reported that zigpy-cli still does not generate valid zigpy indexes.
We need to wrap the firmware list in firmwares (in a dict), see the schema: zigpy/ota/json_schemas.py#L260

This requirement seems to exist since quite a while, possibly introduced with the big OTA v2 PR:

• OTA v2: provider and metadata rewrite zigpy#1340

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 2.61%. Comparing base (1c6d98f) to head (d4ea905).
⚠️ Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
zigpy_cli/ota.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##             dev     #61   +/-   ##
=====================================
  Coverage   2.61%   2.61%           
=====================================
  Files          9       9           
  Lines        613     613           
=====================================
  Hits          16      16           
  Misses       597     597           

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