fix: install fish completions to vendor directory

[Legimity]

Fixes #2850.

This updates the Linux package metadata so the Fish completion file is installed to Fish vendor completions:

• /usr/share/fish/vendor_completions.d/task.fish

The previous package path used /usr/share/fish/completions/task.fish, which is intended for completions shipped with Fish itself.

Validation run:

• ruby -e 'require "yaml"; YAML.load_file(".goreleaser.yml"); YAML.load_file(".goreleaser-pr.yml"); YAML.load_file(".goreleaser-nightly.yml"); puts "yaml ok"'
• ruby -e 'require "yaml"; y=YAML.load_file(".goreleaser.yml"); fish=y.fetch("nfpms").first.fetch("contents").find{|e| e["src"]=="completion/fish/task.fish"}; abort "bad fish dst" unless fish["dst"]=="/usr/share/fish/vendor_completions.d/task.fish"; puts "fish nfpm dst ok"'
• git diff --check

Not run:

• goreleaser check --config .goreleaser.yml: goreleaser is not installed locally.
• go test ./...: local Go is 1.24.3, while this repo currently requires 1.25.10; automatic toolchain download timed out in my environment.

AI assistance disclosure: I used AI assistance to draft and verify this patch, then reviewed the diff and validation output before submitting.

• I have read and followed the Contribution Guide

[trulede]

@Legimity Thanks, looks good.

You should remove the changelog stuff. That gets done by "other means".

[Legimity]

Thanks! I removed the changelog changes and kept this PR focused on the nfpm Fish completion path update.