fix: Fall back to copying temporary preprocessed output file

fix: Fall back to copying temporary preprocessed output file

[1] added a suitable file extension to the temporary file used for
capturing the preprocessed output by creating a hard link. This fails
when the temporary directory is on a file system that doesn't support
hard links.

Fix this by falling back to copying when hard-linking fails.

Closes #1079.

[1]: 4da9f2b474b7ee39cd54ab4261f90936f3c944ec