Make ZYTE_API_ENABLED=False disable the add-on too

[AdrianAtZyte]

Proposed by @VMRuiz so that, when you set ZYTE_API_ENABLED=False to temporarily disable the add-on, you don’t get the download handlers installed anyway.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.04%. Comparing base (197d22c) to head (9d876d6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #324      +/-   ##
==========================================
+ Coverage   97.79%   98.04%   +0.25%     
==========================================
  Files          15       15              
  Lines        2407     2409       +2     
  Branches      439      440       +1     
==========================================
+ Hits         2354     2362       +8     
+ Misses         26       20       -6     
  Partials       27       27              

Files with missing lines	Coverage Δ
scrapy_zyte_api/addon.py	95.91% <100.00%> (+0.17%)	⬆️

... and 1 file with indirect coverage changes

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