[openjdk-11, openjdk-17] d/t/jtreg-autopkgtest.*: force utf-8 for jtreg tests.

openjdk up to 17 uses host encoding by default. openjdk-21 uses utf-8. The tests require utf8, e.g. FAILED: java/net/httpclient/FilePublisher/FilePublisherPermsTest.java[1]

This PR forces utf-8 for jtreg tests.

Changes:

  * d/t/jtreg-autopkgtest.*: force utf-8 encoding.

PPA: ppa:vpa1977/encoding [2]

Testing:

• rebuild in the PPA[2]. Tests pass.

[1] https://launchpadlibrarian.net/821170022/buildlog_ubuntu-questing-s390x.openjdk-17_17.0.17~8ea-1_BUILDING.txt.gz [2] https://launchpad.net/~vpa1977/+archive/ubuntu/encoding/+packages