Set the correct permissions on the zshenv

[Hoverbear]

Description

I'm a bit curious how we didn't catch #254 in testing, since we do that nix is on path:

nix-installer/.github/workflows/ci.yml

Lines 123 to 126 in c150d60

	- name: Test zsh
	run: nix-instantiate -E 'builtins.currentTime' --eval
	if: success() || failure()
	shell: zsh --login --interactive {0}

And that should have failed....

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

[cole-h]

CI, why

[Hoverbear]

Also updated the error message a user might get.