fix(nix): restore install script in fileset for desktop build

[jerome-benoit]

Fixes regression introduced in #12818.

Closes #12817
Closes #12811

Summary

• Restores ../install to the nix fileset in node_modules.nix
• The desktop build requires this file as cli.rs uses include_str!("../../../../install")

Verification

The error reported in #12818 (comment):

error: couldn't read `src/../../../../install`: No such file or directory (os error 2)
  --> src/cli.rs:56:30
   |
56 | const INSTALL_SCRIPT: &str = include_str!("../../../../install");

[github-actions]

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This PR doesn't have a linked issue. All PRs must reference an existing issue.

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[Copilot]

Pull request overview

Restores a missing install script to the Nix fileset used for the desktop build, fixing a regression where Rust include_str! could not find the file.

Changes:

• Adds ../install back into the node_modules.nix fileset.

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[jerome-benoit]

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