Howto: Build platform only and upload artefacts to local maven repo?

[chhh]

Dear all,
I'm trying to set up an environment where I could build a platform-based app without access to Maven Central.
So far I have checked out NB sources and built the platform (on Windows):

1. git clone git@github.com:apache/netbeans.git nb-repo
2. cd nb-repo
3. set JAVA_HOME env var to point to JDK 11 (Temurin)
4. ant build -Dcluster.config=platform
5. ant build-nbms -Dcluster.config=platform <--- this actually fails, see log at the end of this post
6. Do I need NBM Plugin? (https://bits.netbeans.org/mavenutilities/nbm-maven-plugin/index.html)
7. I guess now I need to somehow use the NetBeans Repository Plugin (https://bits.netbeans.org/mavenutilities/nb-repository-plugin/index.html), but I don't understand how to use it. Which POM should this be added to in order to upload the built platform to local maven repo?

Can somebody please provide an example of how to populate the local maven repository with self-built platform artifacts?

---

Failure output from ant build-nbms:

...
-create-license.file:
    [mkdir] Created dir: C:\Users\chhh\source\repos\netbeans-03\platform\janitor\build\nbm-extras\META-INF
[createlicensesummary] All tests passed

-prepare-mandatory-files-for-module:
[createlicensesummary] All tests passed

up-to-date:

-pre-compile:

-do-compile:
   [depend] Deleted 1 out of date files in 0 seconds
 [nb-javac] Compiling 1 source file to C:\Users\chhh\source\repos\netbeans-03\platform\janitor\build\classes
 [nb-javac] Ignoring source, target and bootclasspath as release has been set
[subant-junit] warning: A file for type 'org.netbeans.modules.janitor.Bundle' already exists on the sourcepath or classpath
[subant-junit] error: warnings found and -Werror specified
[subant-junit] 1 error
[subant-junit] 1 warning

BUILD FAILED
C:\Users\chhh\source\repos\netbeans-03\nbbuild\build.xml:322: The following error occurred while executing this line:
C:\Users\chhh\source\repos\netbeans-03\nbbuild\templates\common.xml:207: Compile failed; see the compiler error output for details.

Where can I find "the compiler output"? And where can I unset this "-Werror" flag?