Generating incremental .mar files does not fully adhere to using specified tmp directory

Back then in #23899 we made our incremental .mar files creation setup adhere to a custom temporary directory as /tmp might just be too small for handling large .mar files (diffs).

However, while I can see we still use that custom temporary directory I can see too that /tmp is used. In particular, when dealing with large incremental files due to a major Firefox ESR update this can still cause incrementals generation to fail.

I suspect there is another switch needed deeper down in the incrementals generation stack, that is in Mozilla's scripts.