refact: concatMapAttrs -> flatMapAttrs

This should be slightly easier to read.
We could apply this to all concatMapAttrs calls.

flake.nix

@@ -58,6 +58,18 @@
         "stdenv"
       ];
 
+      /**
+        `flatMapAttrs attrs f` applies `f` to each attribute in `attrs` and
+        merges the results into a single attribute set.
+
+        This can be nested to form a build matrix where all the attributes
+        generated by the innermost `f` are returned as is.
+        (Provided that the names are unique.)
+
+        See https://nixos.org/manual/nixpkgs/stable/index.html#function-library-lib.attrsets.concatMapAttrs
+       */
+      flatMapAttrs = attrs: f: lib.concatMapAttrs f attrs;
+
       forAllSystems = lib.genAttrs systems;
 
       forAllCrossSystems = lib.genAttrs crossSystems;
@@ -319,17 +331,20 @@
         # system, we should reenable this.
         #perlBindings = self.hydraJobs.perlBindings.${system};
       }
-      // lib.concatMapAttrs (nixpkgsPrefix: nixpkgs: 
-          lib.concatMapAttrs (pkgName: pkg:
-            lib.concatMapAttrs (testName: test: {
       # Add "passthru" tests
-              "${nixpkgsPrefix}${pkgName}-${testName}" = test;
-            }) pkg.tests or {}
-          ) nixpkgs.nix-components
-        ) {
+      // flatMapAttrs {
           "" = nixpkgsFor.${system}.native;
           "static-" = nixpkgsFor.${system}.static;
         }
+        (nixpkgsPrefix: nixpkgs: 
+          flatMapAttrs nixpkgs.nix-components
+            (pkgName: pkg:
+              flatMapAttrs pkg.tests or {}
+              (testName: test: {
+                "${nixpkgsPrefix}${pkgName}-${testName}" = test;
+              })
+            )
+        )
       // devFlake.checks.${system} or {}
       );