Fix auxiliary download script not using provided data directory

[djhoese]

Small typo that stopped the auxiliary download script from customizing the location of data to download. This script was supposed to be so simple it didn't need testing... 🤦

• Closes #xxxx
• Tests added

[codecov]

Codecov Report

Merging #1610 (1b2aae7) into master (4928737) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1610      +/-   ##
==========================================
+ Coverage   92.35%   92.38%   +0.03%     
==========================================
  Files         254      254              
  Lines       37409    37421      +12     
==========================================
+ Hits        34548    34571      +23     
+ Misses       2861     2850      -11     

Flag	Coverage Δ
behaviourtests	4.78% <6.66%> (-0.01%)	⬇️
unittests	92.52% <100.00%> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
satpy/aux_download.py	97.43% <100.00%> (+9.40%)	⬆️
satpy/tests/test_data_download.py	100.00% <100.00%> (ø)

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[mraspaud]

LGTM, thanks for adding the test.