Bump to Nix 2.19.3

[Hoverbear]

Description

There is a new release! Yay

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

[aaronmondal]

@Hoverbear @cole-h Would you mind adding the upload to s3 label to this? AFAIU that would allow users of the nix-installer+magic-nix-cache action to pull this in early which fixes DeterminateSystems/magic-nix-cache#32

• Cross-ref Reenable caching for nix workflows TraceMachina/nativelink#631

[Hoverbear]

@aaronmondal done. :) We'll likely merge and release this soon, just been trying to reproduce some Mac issues that were reported.