home-assistant: use remarshal to convert configuration to YAML

HA doesn't mind the configuration being JSON instead of YAML but since YAML is
the official language, use that as it allows users to easily exchange config
data with other parties in the community.

nixos/modules/services/misc/home-assistant.nix

@@ -134,8 +134,10 @@ in {
       description = "Home Assistant";
       after = [ "network.target" ];
       preStart = lib.optionalString (cfg.config != null) ''
-        rm -f ${cfg.configDir}/configuration.yaml
-        ln -s ${configFile} ${cfg.configDir}/configuration.yaml
+        config=${cfg.configDir}/configuration.yaml
+        rm -f $config
+        ${pkgs.remarshal}/bin/json2yaml -i ${configFile} -o $config
+        chmod 444 $config
       '';
       serviceConfig = {
         ExecStart = "${package}/bin/hass --config '${cfg.configDir}'";

nixos/tests/home-assistant.nix

@@ -51,9 +51,9 @@ in {
     startAll;
     $hass->waitForUnit("home-assistant.service");
 
-    # Since config is specified using a Nix attribute set,
-    # configuration.yaml is a link to the Nix store
-    $hass->succeed("test -L ${configDir}/configuration.yaml");
+    # The config is specified using a Nix attribute set,
+    # but then converted from JSON to YAML
+    $hass->succeed("test -f ${configDir}/configuration.yaml");
 
     # Check that Home Assistant's web interface and API can be reached
     $hass->waitForOpenPort(8123);