add ipv6 enabled maven repository
maven central so far doesn't support ipv6 on the primary URL (see
https://issues.sonatype.org/browse/MVNCENTRAL-658 ), it is only enabled
on a dedicated ipv6.repo1.maven.org URL currently.
Add that one to the list of repositories so that ipv6-only hosts can also
fetch the dependencies.
(when using internal ipv4 as well, you'll also need to export
JAVA_OPTS="-Djava.net.preferIPv6Addresses=true"
since otherwise the gradle wrapper fails to download gradle)
Change-Id: I22f0e07fd4e6bb518396f76db20c174e45588372
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/147917
Tested-by: Jenkins
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>