Fish build not reproducible due to non-deterministic search index
# Description:
Currently https://reproducible.archlinux.org/ lists as bad because it can't reproduce the search index in the documentation, because Sphinx builds non-deterministic search indexes when parallelization is enabled for documentation builds.
That's being worked on upstream as far as I can see (e.g. <https://github.com/sphinx-doc/sphinx/issues/11887>), but meanwhile a workaround would be to turn off parallelization for the doc build. Unfortunately, fish upstream hardcodes `-j auto` in its sphinx-build invocation, but that's presumably easy to patch.
I could make a merge request with the corresponding change, but I figured I'd ask first whether patching upstream to make a build reproducible would be acceptable and desirable for Arch Linux.
# Additional info:
* package version(s): 3.7.0-1
* config and/or log files:
* link to upstream bug report, if any:
# Steps to reproduce:
n/a
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