starship: skip one program invocation on each shell init

[anka-213]

Description

Same simplification as #2862, but for fish and ion.

Both of the shell's indirect init functions do identical or equivalent as what we do in the module already.

Fish

❯ starship init ion
eval $(starship init ion --print-full-init)
❯ starship init fish
source (starship init fish --print-full-init | psub)

I'm not sure why the fish code using psub instead of just starship init fish --print-full-init | source, but the latter works perfectly fine when I try piping the full init to source.

See also starship/starship#2637 for discussion about this. The indirection for fish was removed and the reverted because it was incompatible with eval (starship init fish), but that isn't relevant for us.

I haven't actually tried the ion simplification, but since the output is identical to the code in this module, I can't imagine that it could possibly break anything

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
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  nix-shell --pure tests -A run.all.

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[khaneliman]

Please update the tests to match the new behavior.