pixi: add fish shell completion

[zuisong]

Created with brew bump-formula-pr.

release notes

### Highlights
- Transitioned the `run` subcommand to use the `deno_task_shell` for improved cross-platform functionality. More details in the [Deno Task Runner documentation](https://deno.land/manual@v1.35.0/tools/task_runner#task-runner).
- Added an `info` subcommand to retrieve system-specific information understood by `pixi`.
BREAKING CHANGES

[commands] in the pixi.toml is now called [tasks]. (#177)

Details
Added

The pixi info command to get more system information by @wolfv in (#158)
Documentation on how to use the cli by @ruben-arts in (#160)
Use the deno_task_shell to execute commands in pixi run by @baszalmstra in (#173)
Use new solver backend from rattler by @baszalmstra in (#178)
The pixi command command to the cli by @tdejager in (#177)
Documentation on how to use the pixi auth command by @wolfv in (#183)
Use the newest rattler 0.6.0 by @baszalmstra in (#185)
Build with pixi section to the documentation by @tdejager in (#196)

Fixed

Running tasks sequentially when using depends_on by @tdejager in (#161)
Don't add PATH variable where it is already set by @baszalmstra in (#169)
Fix README by @Hofer-Julian in (#182)
Fix Ctrl+C signal in pixi run by @tdejager in (#190)
Add the correct license information to the lockfiles by @wolfv in (#191)

[branchvincent]

thanks! fixed up the commit message to match the PR title

[github-actions]

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