Add missing docker arg for TPU CI.

[zcain117]

Fixes "docker build" requires exactly 1 argument. for the path where $CIRCLE_PR_NUMBER is unset.

[codecov]

Codecov Report

Merging #6393 into master will decrease coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6393      +/-   ##
==========================================
- Coverage   79.40%   79.16%   -0.24%     
==========================================
  Files         148      148              
  Lines       27200    27200              
==========================================
- Hits        21598    21533      -65     
- Misses       5602     5667      +65     

Impacted Files	Coverage Δ
src/transformers/tokenization_xlm.py	16.26% <0.00%> (-66.67%)	⬇️
src/transformers/modeling_tf_flaubert.py	24.53% <0.00%> (-63.20%)	⬇️
src/transformers/tokenization_openai.py	71.21% <0.00%> (-11.37%)	⬇️
src/transformers/tokenization_transfo_xl.py	33.56% <0.00%> (-5.17%)	⬇️
src/transformers/tokenization_utils.py	90.40% <0.00%> (+0.40%)	⬆️
src/transformers/tokenization_bert.py	91.51% <0.00%> (+0.44%)	⬆️
src/transformers/tokenization_utils_fast.py	94.28% <0.00%> (+2.14%)	⬆️
src/transformers/generation_tf_utils.py	86.46% <0.00%> (+7.26%)	⬆️
src/transformers/tokenization_utils_base.py	93.84% <0.00%> (+7.41%)	⬆️
src/transformers/tokenization_roberta.py	98.63% <0.00%> (+21.91%)	⬆️
... and 1 more

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[LysandreJik]

Great, LGTM