Multiple ':envvar:' check warnings in Doc/using/windows.rst

[terryjreedy]

PR #103745 changed a spelling in Doc/using/windows.rst. Upon encountering texts of the form ":envvar:`XYZ`", GitHub Actions / Docs emitted several checkwarnings of the form 'envvar' reference target not found: XYZ where XYZ is PATH (multiple times), PATHEXT, TEMP, or %APPDATA%. I suspect other docs might have the same issues.

[aelsayed95]

Yes, running sphinx in nitpicky mode, I similar errors in many files where :envvar: is used. It seems many env vars are missing definitions.

For windows.rst specifically, I see two types of undefined env-vars that generate errors:

1. Generic env vars: PATH, TEMP, %APPDATA%
2. Env vars that are related to the windows python launcher or related to windows in general: PYLAUNCHER_DRYRUN, PYLAUNCHER_ALLOW_INSTALL, PYLAUNCHER_ALWAYS_INSTALL, PYLAUNCHER_DEBUG, PATHEXT

Group 2 needs definitions to allow :envvar: to link correctly to the respective env var's usage. Maybe a definition section can be added to Doc/using/windows.rst

For Group 1, it seems reasonable to just omit the :envvar: directive so that Sphinx doesn't attempt to find references for and just use double ticks instead around the env var name instead?

[AA-Turner]

Closing as a duplicate of #101100.