build: remove distutils dependency

[beersandrew]

Description of Change(s)

• swap out usages of find_executable with shutil.which
• replace strtobool with an equivalent function (https://docs.python.org/3.9/distutils/apiref.html#distutils.util.strtobool)

Link to proposal (if applicable)

Fixes Issue(s)

• https://github.com/PixarAnimationStudios/OpenUSD/issues/3389

Checklist

[X] I have created this PR based on the dev branch

[X] I have followed the coding conventions

[ ] I have added unit tests that exercise this functionality (Reference: testing guidelines)

[ ] I have verified that all unit tests pass with the proposed changes

[X] I have submitted a signed Contributor License Agreement (Reference: Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-10422

[jesschimein]

/AzurePipelines run

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