Set Python_EXECUTABLE env var in GHA workflows

[mitchute]

Pull request overview

• Some of the GHA workflows fail due to a missing Python_EXECUTABLE env var. This sets it explicitly.

Description of the purpose of this PR

Pull Request Author

• Title of PR should be user-synopsis style (clearly understandable in a standalone changelog context)
• Label the PR with at least one of: Defect, Refactoring, NewFeature, Performance, and/or DoNoPublish
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• If structural output changes, add to output rules file and add OutputChange label
• If adding/removing any LaTeX docs or figures, update that document's CMakeLists file dependencies
• If adding/removing any output files (e.g., eplustbl.*)
  • Update ..\scripts\Epl-run.bat
  • Update ..\scripts\RunEPlus.bat
  • Update ..\src\EPLaunch\ MainModule.bas, epl-ui.frm, and epl.vbp (VersionComments)
  • Update ...github\workflows\energyplus.py

Reviewer

• Perform a Code Review on GitHub
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• If defect, verify by running develop branch and reproducing defect, then running PR and reproducing fix
• If feature, test running new feature, try creative ways to break it
• CI status: all green or justified
• Check that performance is not impacted (CI Linux results include performance check)
• Run Unit Test(s) locally
• Check any new function arguments for performance impacts
• Verify IDF naming conventions and styles, memos and notes and defaults
• If new idf included, locally check the err file and other outputs

[mitchute]

GHA workflows completed successfully, and I'm not waiting for Decent CI since this will have no effect there. Merging.