Export mock aws credentials for s3 tests

[ayushdg]

Fixes issue for failing s3 tests on machines missing aws credentials.
The PR exports fake credentials to prevent botocore from looking for aws credentials on the machine.

[kkraus14]

@gpucibot merge

[codecov]

Codecov Report

Merging #7176 (40b42d3) into branch-0.18 (8860baf) will increase coverage by 0.08%.
The diff coverage is 85.18%.

@@               Coverage Diff               @@
##           branch-0.18    #7176      +/-   ##
===============================================
+ Coverage        82.09%   82.17%   +0.08%     
===============================================
  Files               97       97              
  Lines            16474    16543      +69     
===============================================
+ Hits             13524    13594      +70     
+ Misses            2950     2949       -1     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/lists.py	91.66% <ø> (-0.09%)	⬇️
python/cudf/cudf/core/dataframe.py	90.71% <50.00%> (+<0.01%)	⬆️
python/cudf/cudf/io/orc.py	86.80% <60.00%> (-1.61%)	⬇️
python/cudf/cudf/core/dtypes.py	89.50% <66.66%> (-0.88%)	⬇️
python/cudf/cudf/io/csv.py	91.66% <77.77%> (-1.67%)	⬇️
python/cudf/cudf/core/series.py	91.10% <80.00%> (-0.06%)	⬇️
python/cudf/cudf/core/reshape.py	91.00% <81.81%> (-0.04%)	⬇️
python/cudf/cudf/core/frame.py	89.90% <88.88%> (-0.08%)	⬇️
python/dask_cudf/dask_cudf/io/tests/test_s3.py	95.94% <94.11%> (+0.11%)	⬆️
python/cudf/cudf/core/column/numerical.py	94.08% <100.00%> (-0.33%)	⬇️
... and 15 more

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