fix(nix): expose libstdc++ during completion install

[jamesbrink]

Summary

• add a Linux-only preInstall hook to expose stdenv.cc.cc.lib on LD_LIBRARY_PATH
• keeps the CUDA-free cargoWithProfile run ... completions helper runnable before Nix fixup patches runtime paths
• document the Nix sandbox completion-generation fix in the changelog

Closes #336.

Validation

• nix fmt
• nix eval --raw .#packages.x86_64-linux.mold.preInstall
• nix build .#packages.x86_64-linux.mold --dry-run
• git diff --check
• nix flake check --all-systems --no-build
• codex review --uncommitted (no findings)

[greptile-apps]

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[Copilot]

Pull request overview

This PR fixes a Nix sandbox build failure on Linux where shell completion generation in postInstall runs the freshly built (CUDA-free) target/release/mold before Nix fixup has patched runtime paths, causing a dynamic loader error for libstdc++.so.6.

Changes:

• Add a Linux-only preInstall hook to export stdenv.cc.cc.lib onto LD_LIBRARY_PATH so completion generation can execute in sandboxed builders.
• Document the Nix sandbox completion-generation fix in the changelog (issue #336).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
flake.nix	Adds Linux-only preInstall that prepends stdenv.cc.cc.lib to LD_LIBRARY_PATH so postInstall completion generation can run in Nix sandbox.
CHANGELOG.md	Records the Linux Nix sandbox completion-generation fix and links to #336.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.91%. Comparing base (955943d) to head (da29b18).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #337      +/-   ##
==========================================
- Coverage   66.91%   66.91%   -0.01%     
==========================================
  Files         254      254              
  Lines      142741   142741              
==========================================
- Hits        95519    95518       -1     
- Misses      47222    47223       +1     

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