Replace pkgs.hostPlatform by pkgs.stdenv.hostPlatform

We are attempting to deprecate these top-level attributes in upstream
Nixpkgs.

See also bc0acdad8c.

modules/config/i18n.nix

@@ -36,7 +36,7 @@ let
 in {
   meta.maintainers = with maintainers; [ midchildan ];
 
-  config = mkIf pkgs.hostPlatform.isLinux {
+  config = mkIf pkgs.stdenv.hostPlatform.isLinux {
     # For shell sessions.
     home.sessionVariables = localeVars;
 

modules/misc/xdg-mime.nix

@@ -10,7 +10,7 @@ in {
   options = {
     xdg.mime.enable = mkOption {
       type = types.bool;
-      default = pkgs.hostPlatform.isLinux;
+      default = pkgs.stdenv.hostPlatform.isLinux;
       defaultText =
         literalExpression "true if host platform is Linux, false otherwise";
       description = ''

modules/services/window-managers/xmonad.nix

@@ -132,9 +132,9 @@ in {
 
           # The resulting binary name depends on the arch and os
           # https://github.com/xmonad/xmonad/blob/56b0f850bc35200ec23f05c079eca8b0a1f90305/src/XMonad/Core.hs#L565-L572
-          mv "$XMONAD_DATA_DIR/xmonad-${pkgs.hostPlatform.system}" $out/bin/
+          mv "$XMONAD_DATA_DIR/xmonad-${pkgs.stdenv.hostPlatform.system}" $out/bin/
         ''
-      }/bin/xmonad-${pkgs.hostPlatform.system}";
+      }/bin/xmonad-${pkgs.stdenv.hostPlatform.system}";
 
   in mkIf cfg.enable (mkMerge [
     {
@@ -157,7 +157,7 @@ in {
     (mkIf (cfg.config != null) {
       xsession.windowManager.command = xmonadBin;
       home.file.".xmonad/xmonad.hs".source = cfg.config;
-      home.file.".xmonad/xmonad-${pkgs.hostPlatform.system}" = {
+      home.file.".xmonad/xmonad-${pkgs.stdenv.hostPlatform.system}" = {
         source = xmonadBin;
         onChange = ''
           # Attempt to restart xmonad if X is running.

modules/targets/darwin/fonts.nix

@@ -12,7 +12,7 @@ let
   fonts = "${fontsEnv}/share/fonts";
 in {
   # macOS won't recognize symlinked fonts
-  config = mkIf pkgs.hostPlatform.isDarwin {
+  config = mkIf pkgs.stdenv.hostPlatform.isDarwin {
     home.activation.copyFonts = hm.dag.entryAfter [ "writeBoundary" ] ''
       copyFonts() {
         rm -rf ${homeDir}/Library/Fonts/HomeManager || :