buildFHSEnv: bind /nix as read-write

[james-atkins]

Tweak bubblewrap options so /nix is mounted read-write rather than read-only: this is required for running a proprietary installer that requires an FHS environment. Ran into this when trying to package Matlab (#356831).

Fixes #239017.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[philiptaron]

I'm a little loathe to merge this. It's a pretty big increase in what applications are hypothetically able to do. Shouldn't this be a derivation-by-derivation decision?

[james-atkins]

The nix store is mounted read-only at a lower level so this change shouldn't affect what wrapped applications are able to do, I think. But I would be happy to add a per-derivation flag.