nixos/nexus: add jvm package option

nixos/modules/services/web-apps/nexus.nix

@@ -7,7 +7,6 @@ let
   cfg = config.services.nexus;
 
 in
-
 {
   options = {
     services.nexus = {
@@ -19,6 +18,16 @@ in
         description = "Package which runs Nexus3";
       };
 
+      jdkPackage = mkOption {
+        type = types.package;
+        default = pkgs.openjdk8;
+        defaultText = literalExample "pkgs.openjdk8";
+        example = literalExample "pkgs.openjdk8";
+        description = ''
+          The JDK package to use.
+        '';
+      };
+
       user = mkOption {
         type = types.str;
         default = "nexus";
@@ -101,6 +110,7 @@ in
         NEXUS_USER = cfg.user;
         NEXUS_HOME = cfg.home;
 
+        INSTALL4J_JAVA_HOME = "${cfg.jdkPackage}";
         VM_OPTS_FILE = pkgs.writeText "nexus.vmoptions" cfg.jvmOpts;
       };