PlaceNixConfiguration: don't write ssl-cert-file to nix.custom.conf if determinate and macOS

[cole-h]

determinate-nixd will handle the ssl-cert-file setting in its managed nix.conf, which will override the ssl-cert-file that may already be configured in the user's nix.custom.conf.

This is NOT TRUE for non-Determinate installs, since the !include nix.custom.conf statement is at the bottom of nix.conf on non-Determinate installs, meaning it will override whatever is set in nix.conf; for Determinate installs, the include statement is at the top of nix.conf, meaning the Determinate-managed config will override anything set in the nix.custom.conf (such as ssl-cert-file).

Description

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