Set default ComSpec when running on Windows

[pingsutw]

TL;DR

Fixes below errors in the CI, The test is failing because the latest python requires user to set COMSPEC in environment variable.
For more detail, check python/cpython#101283

FAILED tests/flytekit/unit/extras/tasks/test_shell.py::test_shell_task_no_io - FileNotFoundError: shell not found: neither %ComSpec% nor %SystemRoot% is set
FAILED tests/flytekit/unit/extras/tasks/test_shell.py::test_input_substitution_primitive - FileNotFoundError: shell not found: neither %ComSpec% nor %SystemRoot% is set
FAILED tests/flytekit/unit/extras/tasks/test_shell.py::test_input_substitution_files - FileNotFoundError: shell not found: neither %ComSpec% nor %SystemRoot% is set
FAILED tests/flytekit/unit/extras/tasks/test_shell.py::test_input_substitution_files_ctx - FileNotFoundError: shell not found: neither %ComSpec% nor %SystemRoot% is set
FAILED tests/flytekit/unit/extras/tasks/test_shell.py::test_input_output_substitution_files - FileNotFoundError: shell not found: neither %ComSpec% nor %SystemRoot% is set
FAILED tests/flytekit/unit/extras/tasks/test_shell.py::test_input_single_output_substitution_files - FileNotFoundError: shell not found: neither %ComSpec% nor %SystemRoot% is set
FAILED tests/flytekit/unit/extras/tasks/test_shell.py::test_reuse_variables_for_both_inputs_and_outputs - FileNotFoundError: shell not found: neither %ComSpec% nor %SystemRoot% is set
FAILED tests/flytekit/unit/extras/tasks/test_shell.py::test_can_use_complex_types_for_inputs_to_f_string_template - FileNotFoundError: shell not found: neither %ComSpec% nor %SystemRoot% is set

Type

• Bug Fix
• Feature
• Plugin

Are all requirements met?

• Code completed
• Smoke tested
• Unit tests added
• Code documentation added
• Any pending items have an associated Issue

Complete description

^^^

Tracking Issue

NA

Follow-up issue

NA

[codecov]

Codecov Report

Merging #1595 (d138eb6) into master (e865db5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1595   +/-   ##
=======================================
  Coverage   70.04%   70.04%           
=======================================
  Files         326      326           
  Lines       29913    29916    +3     
  Branches     5391     5392    +1     
=======================================
+ Hits        20952    20955    +3     
  Misses       8437     8437           
  Partials      524      524           

Impacted Files	Coverage Δ
flytekit/extras/tasks/shell.py	78.08% <100.00%> (+0.37%)	⬆️

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[eapolinario]

This feels really hacky. No one runs windows containers in flyte, so I don't feel that bad about it.

In other words, we're making this change just to have consistency in running tests.