execute_process: Improve invocation of .cmd/.bat with spaces

Extend the fix from commit 74c9d40876 (execute_process: Fix invocation
of .cmd/.bat with spaces, 2025-01-31) to work without relying on
conversion to a "short path", which may not exist.  Instead, extending
the `cmd /c` wrapper to `cmd /c call` seems to support spaces directly.

Suggested-by: Alexandru Croitor <alexandru.croitor@qt.io>
Fixes: #26655