signald: set jdkOnBuild

This is required if the building jdk is a different version than what is default
which is now 21.

Author: Atemu

pkgs/by-name/si/signald/package.nix

@@ -25,6 +25,7 @@ let
 
   jre' = jre_minimal.override {
     jdk = jdk17_headless;
+    jdkOnBuild = jdk17_headless;
     # from https://gitlab.com/signald/signald/-/blob/0.23.0/build.gradle#L173
     modules = [
       "java.base"