ARROW-2795: [Python] Run TensorFlow import workaround only on Linux platforms

[wesm]

Per comments in #2210

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@037c156). Click here to learn what that means.
The diff coverage is 41.66%.

@@            Coverage Diff            @@
##             master    #2218   +/-   ##
=========================================
  Coverage          ?   86.52%           
=========================================
  Files             ?      232           
  Lines             ?    41089           
  Branches          ?        0           
=========================================
  Hits              ?    35554           
  Misses            ?     5535           
  Partials          ?        0

Impacted Files	Coverage Δ
python/pyarrow/__init__.py	64.28% <41.66%> (ø)

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[wesm]

+1