Fix fish support on ostree

[grahamc]

Description

Closes #707

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

[grahamc]

Unfortunately this doesn't work: the plan says to create the directory, then the planning phase detects the directory doesn't exist (yet) and decides to not install it. Hrm.

[Hoverbear]

I believe that is because of

nix-installer/src/action/common/configure_shell_profile.rs

Lines 83 to 86 in a026cc2

	if !fish_prefix_path.exists() {
	// If the prefix doesn't exist, don't create the `conf.d/nix.fish`
	continue;
	}

Perhaps that should happen at execute time.

[cole-h]

Marking this as a draft based on this previous comment: #711 (comment).