* Add flake.variants
This allows flakes to easily include variations of the
project by with different project arguments.
Anything you can pass to `project.addModule` can be used.
For instance to include variants using ghc 9.2.4:
```
flake.variants.ghc924.compiler-nix-name = pkgs.lib.mkForce "ghc924";
```
Then use it with:
```
nix build .#ghc924:hello:exe:hello
```
This change also:
* Exposes add `combineFlakes` and `rawFlake` for more control over how flakes are combined.
* Includes package coverageReports to the `hydraJobs` of the flakes.
* Tries to make the materialisation concurrency test more reliable
Fixes:
Warning: `stdenv.lib` is deprecated and will be removed in the next release. Please use `pkgs.lib` instead. For more information see https://github.com/NixOS/nixpkgs/issues/108938
Adding materialize call to a derivation should not prevent nix form
building it concurrently with other derivations.
This test makes sure that is the case.
Hamish Mackenzie
Hamish Mackenzie