runPhase subshell and exit status

[mitchallain]

This PR follows up on #330751, modifying the runPhase function to be more useful for command chaining and scripting in devshells. I will restate the context below so this PR is self-sufficient, but newcomers are encouraged to review that PR and comment thread for additional context.

The relatively new runPhase function in stdenv seems to be the best practice for building packages within a devshell, rather than manually executing sub-commands (e.g., cmake .. && make). This creates better consistency between dev workflow and the eventual realized derivation created by nix build.

However, the runPhase command obscures any failures which take place within the individual build phases. To fix that problem, this PR sets the errexit / set -e shell option so that errors within simple commands, lists, or compound commands in the individual build phases will cause the phase to exit immediately, and cause runPhase to return non-zero. This is consistent with the behavior of the stdenv builder which uses the set -e option.

A subshell is used to prevent setting the errexit option in the devshell, causing the devshell to exit on failure, which is generally not preferred.

This allows workflows like chaining commands in lists (e.g., runPhase configurePhase && runPhase buildPhase) or scripting phases in the devshell (trivial example: runPhase buildPhase && echo "Succeeded" || echo "Failure").

Caveat: using a subshell means that phases cannot modify variables in the devshell, which could be breaking or could (?) be beneficial. I'll convert this PR to "Ready for Review" once this is better understood.

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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