Run shellcheck on nix-installer.sh

[lucperkins]

Description

At the moment, we do have a shellcheck directive in nix-installer.sh but we don't actually run shellcheck against it. Doing so has exposed two very minor infractions but worth automating this.

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]

This script is lightly edited from the rustup init script (https://github.com/rust-lang/rustup/blob/master/rustup-init.sh). Maybe you could also send this diff upstream to them? Their script is fairly robust, so it would be sad to drift terribly far from what they're doing just to satisfy shellcheck. (I know this change isn't that much of a diff, but if shellcheck ever becomes unhappy about larger parts...)

[lucperkins]

@cole-h Okay, I've determined that we actually do not want to quote $_retry here so I've added disable statements instead.