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Fix T72214: Fluids: noise does not work with negative frame numbers

The issue is duplicated code. There are two functions that zero-fill
the frame number. They worked the same for positive frames numbers, but
behaved differently for negative ones.

On frame `-100`, `BLI_path_frame` outputs `-0100` and
`fluid_cache_get_framenr_formatted_$ID$` outputted `-100`.

I changed the behavior of the latter, because we depend on the behavior
of the former for much longer already.

Reviewers: sebbas

Differential Revision: https://developer.blender.org/D8107