fix: completion generation for Fish

[mmartinortiz]

First things first: Thank you for creating this feature, I find it quite useful

I'm a Fish Shell user, and I noted that the uv completion was not working, after looking at the code and test in my computer, I solved it removing the source command from the pipe.

Here is the template that I used to test the feature as it is right now:

{
	"name": "Python",
	"image": "mcr.microsoft.com/devcontainers/python:1-3.13-bookworm",
	"features": {
		"ghcr.io/va-h/devcontainers-features/uv:1": {"shellautocompletion": true},
		"ghcr.io/meaningful-ooo/devcontainer-features/fish:1": {}
	}
}

The following command did not work on an out of the box fish shell:

uv generate-shell-completion fish | source >> /home/vscode/.config/fish/config.fish

But this one did:

uv generate-shell-completion fish >> /home/vscode/.config/fish/config.fish