tests.nixpkgs-check-by-name: Set evaluation system to x86_64-linux

This was previously a checking impurity that could produce different
results when run on different systems.

pkgs/test/nixpkgs-check-by-name/README.md

@@ -28,6 +28,8 @@ These checks are performed by this tool:
 - Each package directory must not refer to files outside itself using symlinks or Nix path expressions.
 
 ### Nix evaluation checks
+
+Evaluate Nixpkgs with `system` set to `x86_64-linux` and check that:
 - For each package directory, the `pkgs.${name}` attribute must be defined as `callPackage pkgs/by-name/${shard}/${name}/package.nix args` for some `args`.
 - For each package directory, `pkgs.lib.isDerivation pkgs.${name}` must be `true`.
 

pkgs/test/nixpkgs-check-by-name/src/eval.nix

@@ -54,6 +54,9 @@ let
     # Don't let the users home directory influence this result
     config = { };
     overlays = [ overlay ];
+    # We check evaluation and callPackage only for x86_64-linux.
+    # Not ideal, but hard to fix
+    system = "x86_64-linux";
   };
 
   attrInfo = name: value:

pkgs/test/nixpkgs-check-by-name/tests/mock-nixpkgs.nix

@@ -19,6 +19,8 @@ It returns a Nixpkgs-like function that can be auto-called and evaluates to an a
   overlays ? [],
   # Passed by the checker to make sure a real Nixpkgs isn't influenced by impurities
   config ? {},
+  # Passed by the checker to make sure a real Nixpkgs isn't influenced by impurities
+  system ? null,
 }:
 let