qt6.qtwebengine: correct how NIX_BUILD_CORES is propagated to the gn …

[NickCao]

…ninja

port of #37889

should fix the current build failure on hydra.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 22.11 Release Notes (or backporting 22.05 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/why-is-qt6-qtwebengine-failing-ulimit-n/19539/15

[NickCao]

The 16 cores limit is arbitrary and may not be required, just trying to play it safe.

[LunNova]

Would it make sense to update the ninja setup hook in a future PR to append this to NINJAFLAGS instead of only doing it when it invokes it? That way it'd help for other projects which have cmake invoking ninja like this one.

nixpkgs/pkgs/development/tools/build-managers/ninja/setup-hook.sh

Line 8 in 0badfc4

buildCores="$NIX_BUILD_CORES"

[LunNova]

Testing with nix build -j 1 --cores 1 .#qt6Packages.qtwebengine, at times it still used many cc instances for the first build step. The build step with 30k steps does seem to be fixed.

qtwebengine> [5/253] Performing build step for 'gn'
qtwebengine> [0/1] Generating gn
qtwebengine> ninja: Entering directory `/build/qtwebengine-everywhere-src-6.4.0/build/src/gn/Release'
qtwebengine> [1/195] CXX src/base/third_party/icu/icu_utf.o
...

This was the section that still used 32 cores.

[K900]

Should we just merge and see what happens? People are hitting this a bunch now, and it'll only get worse as more things move to qt6.

[nrdxp]

Just merged #192778 and now I'll merge this as well and hopefully the build will finally pass 🤞

update
The build is queued