Revert "fix: Fall back to copying temporary preprocessed output file"

Revert "fix: Fall back to copying temporary preprocessed output file"

This reverts commit cbedccc9fa30f8958dc3e7fbdf900a952f63d5ad since the
fix doesn't work.

The hard link in question is used in get_result_key_from_cpp to create
an alias of file that captures stdout from the preprocessor, where the
alias has the correct cpp extension. It can't just be a copy since
do_execute writes to the original name. See #1079.