headscale.oidc: client_secret_path is a string

It can be include an environment-variable, like
`${CREDENTIALS_DIRECTORY}/some-path`, failing validation for
`types.path`.

nixos/modules/services/networking/headscale.nix

@@ -292,7 +292,7 @@ in {
               };
 
               client_secret_path = mkOption {
-                type = types.nullOr types.path;
+                type = types.nullOr types.str;
                 default = null;
                 description = lib.mdDoc ''
                   Path to OpenID Connect client secret file. Expands environment variables in format ''${VAR}.