Repair is_ci handling, sudo was erasing the variables

[Hoverbear]

Description

sudo was erasing the variables which is_ci checks for CI. Instead of preserving the entire list, we opt to run it once and set a nix-installer specific env for CI and or with is_ci's result later.

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]

Appears to work fine!