flake: gate development outputs to supported systems

Keep the home-manager package exposed for all nixpkgs flakeExposed systems, but only expose formatter, devShell, docs, tests, CI helpers, and legacy test packages on supported Home Manager systems.

This prevents nix flake check --all-systems from forcing unavailable formatter toolchains such as GHC on riscv64-linux and x86_64-freebsd.

Fixes #9389.

flake.nix

@@ -45,9 +45,19 @@
     }
     // (
       let
-        forAllPkgs =
-          f:
-          nixpkgs.lib.genAttrs nixpkgs.lib.systems.flakeExposed (system: f nixpkgs.legacyPackages.${system});
+        supportedSystems = [
+          "aarch64-darwin"
+          "aarch64-linux"
+          "i686-linux"
+          "x86_64-darwin"
+          "x86_64-linux"
+        ];
+
+        forSystems = systems: f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
+
+        forAllPkgs = forSystems nixpkgs.lib.systems.flakeExposed;
+
+        forSupportedPkgs = forSystems supportedSystems;
 
         forCI = nixpkgs.lib.genAttrs [
           "aarch64-darwin"
@@ -156,7 +166,7 @@
           lib.mapAttrs' renameTestPkg tests;
       in
       {
-        formatter = forAllPkgs (pkgs: pkgs.callPackage ./home-manager/formatter.nix { });
+        formatter = forSupportedPkgs (pkgs: pkgs.callPackage ./home-manager/formatter.nix { });
 
         # TODO: increase buildbot testing scope
         buildbot = forCI (
@@ -187,6 +197,8 @@
           {
             default = hmPkg;
             home-manager = hmPkg;
+          }
+          // nixpkgs.lib.optionalAttrs (nixpkgs.lib.elem pkgs.stdenv.hostPlatform.system supportedSystems) {
 
             ci-parse = pkgs.callPackage ./ci/parse.nix { nix = pkgs.nixVersions.latest; };
             ci-parse-lix = pkgs.callPackage ./ci/parse.nix {
@@ -212,11 +224,11 @@
           }
         );
 
-        devShells = forAllPkgs (pkgs: {
+        devShells = forSupportedPkgs (pkgs: {
           default = pkgs.callPackage ./home-manager/devShell.nix { };
         });
 
-        legacyPackages = forAllPkgs (
+        legacyPackages = forSupportedPkgs (
           pkgs:
           let
             inherit (pkgs.stdenv.hostPlatform) system;