bug#55776: maven-core fails to build

[Remco van 't Veer <remco@remworks.net>]

I did some digging and found this regression is caused by commit:

 6068b83b82475566acd4162467bcf54270f338f9
 "gnu: java-jdom: Update to 2.0.6.1 [fixes CVE-2021-33813]."

Apparently the fix for this issue causes jdom to be very strict;

> java.io.IOException: Invalid input descriptor for merge:
> /tmp/plexus-metadata3957336728290309540xml -->
> http://xml.org/sax/features/external-general-entities feature
> http://xml.org/sax/features/external-general-entities not supported
> for SAX driver org.codehaus.plexus.metadata.merge.Driver

Which sound familiar when looking at that CVE
(https://github.com/advisories/GHSA-2363-cqg2-863c):

> An XXE issue in SAXBuilder in JDOM through 2.0.6 allows attackers to
> cause a denial of service via a crafted HTTP request. At this time
> there is not released fixed version of JDOM. As a workaround, to avoid
> external entities being expanded, one can call
> builder.setExpandEntities(false) and they won't be expanded.

I dunno how to fix this though, I'm just a curious guixer.  Easiest path
seems to be to make a new java-jdom-2.0.6 var and use that as a
native-input for maven.  Would that be an acceptable solution?

Cheers,
Remco