build : fix hysteresis with system-test

Motivation:

A clean build-and-test, and some targeted partial builds, can fail when the `system-test` test is executed. This can lead to unwanted failures in automatic builds, longer build-fix-test cycles, and AI confusion during agentic AI conversations and updates.

I asked Copilot using GPT-5 in agent mode to analyze the build system configuration and the composition of the test, and attempt to diagnose and resolve the issue. Per the AI session log:

> ## Summary
>
> On a clean build, the "System tests" module (packages/system-test) could fail because it depends on an un-attached ZIP artifact (org.dcache:dcache-plugin-hsqldb:zip). The HSQLDB plugin produced its ZIP via the assembly plugin but didn’t attach it, so a first-run reactor build couldn’t resolve the ZIP until a prior install populated the local repository (commonly by running with -DskipTests). Attaching the ZIP fixes first-run builds.
>
>
> ## Root Cause
>
> - Module: plugins/hsqldb (artifactId: dcache-plugin-hsqldb)
> - It builds a ZIP using maven-assembly-plugin but didn’t attach that ZIP to the project artifacts.
> - Downstream: packages/system-test declares a dependency on dcache-plugin-hsqldb with type zip and unpacks it.
> - On a clean machine/repo, the ZIP isn’t in the local repository and not available to the reactor resolver unless it’s attached or previously installed.

Modificaton:

A one-line addition to `plugins/hsqldb/pom.xml` attaches the generated ZIP file to the project artifacts, allowing Maven to track it and ensure that it is built prior to the execution of the system test.

Target: master
Patch: https://rb.dcache.org/r/14527/
Requires-notes: NO
Requires-book: NO
Acked-by: Tigran Mkrtchyan

Author: greenc-FNAL

plugins/hsqldb/pom.xml

@@ -31,6 +31,7 @@
                       <format>zip</format>
                   </formats>
                   <appendAssemblyId>false</appendAssemblyId>
+                  <attach>true</attach>
               </configuration>
               <executions>
                   <execution>