Groovy @Grab: grapes are downloaded in servlet container's home directory

We have a development instance running XWiki 11.10.10, it runs jetty as shipped in debian buster. The home directory is /usr/share/jetty9/ and owned by root.

When using @Grab, we have the following error:

Error grabbing grapes – /usr/share/jetty9/.groovy/grapes/resolved-caller-all-caller-working89.xml (No such file or directory)
{{ CAUSED BY}}
{{ @Grab(group="org.codehaus.groovy.modules.http-builder", module="http-builder", version="0.7.1") import groovyx.net.http.HTTPBuilder;}}

Groovy's behavior is documented at http://docs.groovy-lang.org/latest/html/documentation/grape.html#Grape-Detail

(see last line)

The workaround is to install the files from the extension manager.

Note, tomcat user from debian package has a homedir of /

We are wondering why those files aren't downloaded to the Jetty's TMPDIR, which folder is well set to a specific folder - outside /usr/share - on this instance.