Fix #79254: getenv() w/o arguments not showing changes

To be able to see changes done only with `SetEnvironmentVariable()`, we
have to use `GetEnvironmentStrings()` instead of `environ`,

Fix #79254: getenv() w/o arguments not showing changes

To be able to see changes done only with `SetEnvironmentVariable()`, we
have to use `GetEnvironmentStrings()` instead of `environ`, because the
latter sees only changes done with `putenv()`.

For best backward compatibility we're using `GetEnvironmentStringsA()`;
switching to the wide string version likely makes sense for master,
though.

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