nixos/frigate: Add preCheckConfig

Without this, many use cases will fail due to missing env vars for
secrets. This is inspired by e.g. `networking.nftables.preCheckRuleset`.

nixos/modules/services/video/frigate.nix

@@ -53,6 +53,7 @@ let
         cp ${configFileUnchecked} $out
         export CONFIG_FILE=$out
         export PYTHONPATH=${cfg.package.pythonPath}
+        ${cfg.preCheckConfig}
         ${cfg.package.python.interpreter} -m frigate --validate-config || error
       '';
   configFile = if cfg.checkConfig then configFileChecked else configFileUnchecked;
@@ -204,6 +205,16 @@ in
       '';
     };
 
+    preCheckConfig = mkOption {
+      type = types.lines;
+      default = "";
+      description = ''
+        This script gets run before the config is checked. It can be used to,
+        e.g., set environment variables needed or transform the config
+        (available as `$out`) to make it checkable in the sandbox.
+      '';
+    };
+
     settings = mkOption {
       type = submodule {
         freeformType = format.type;