flake: lock base16-fish input to custom patch

[trueNAHO]

Lock the base16-fish input to a custom patch [2] ("Make autosuggestion
and comment base03"), since it is currently impossible to apply patches
to flake inputs [1] ("Support flake references to patches").

Once this single-patch approach no longer scales, the repository should
be properly forked, if [2] has still not been merged.

[1]: https://github.com/NixOS/nix/issues/3920
[2]: https://github.com/tomyun/base16-fish/pull/12

Closes: https://github.com/nix-community/stylix/issues/1730

Might be worth testing this PR with several themes to see if it really improves the situation.

CC: @NovaViper, @gaykitty

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[NovaViper]

It didn't seem to make a difference on my side sadly 😭

[trueNAHO]

It didn't seem to make a difference on my side sadly 😭

I did not actually test this yet with several themes, but the patch should at least have been pulled, as the inputs.base16-fish.outPath tree seems to be the correct one.