testing: Fix build on windows 2019

[tmatsuo]

I need to use the full path of cmd in order to run 7-zip in both images. Probably because some difference between cygwin on windows 2016 image and msys2 on windows 2019 image.

Do we want to keep builds on windows 2016? If not, I'll eventually remove files for windows 2016 builds.

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This change is 

[codecov]

Codecov Report

Merging #2932 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2932      +/-   ##
==========================================
- Coverage   90.58%   90.58%   -0.01%     
==========================================
  Files         297      297              
  Lines       20153    20155       +2     
==========================================
+ Hits        18256    18257       +1     
- Misses       1897     1898       +1

Impacted Files	Coverage Δ
google/cloud/storage/internal/curl_handle.h	65.62% <0%> (-12.5%)	⬇️
google/cloud/storage/well_known_headers.h	77.77% <0%> (-11.12%)	⬇️
google/cloud/storage/internal/generic_request.h	78.37% <0%> (-8.11%)	⬇️
google/cloud/storage/object_stream.h	94.64% <0%> (-1.79%)	⬇️
google/cloud/storage/internal/logging_client.cc	97.76% <0%> (-0.02%)	⬇️
google/cloud/bigtable/row_range.h	100% <0%> (ø)	⬆️
google/cloud/internal/retry_policy.h	100% <0%> (ø)	⬆️
google/cloud/storage/well_known_parameters.h	75.55% <0%> (+4.44%)	⬆️
...oogle/cloud/bigtable/internal/unary_client_utils.h	100% <0%> (+17.85%)	⬆️
google/cloud/storage/oauth2/credentials.h	66.66% <0%> (+33.33%)	⬆️

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[tmatsuo]

Using full path didn't work on windows 2019. Closing for now.

[tmatsuo]

It works when setting Path env var in the powershell script. Reopening

[coryan]

Do we want to keep builds on windows 2016? If not, I'll eventually remove files for windows 2016 builds.

No. We may want to have multiple builds with different MSVC versions, but the base OS is not that relevant I think.

Reviewed 2 of 3 files at r1, 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved