Cirrus CI: Set LANG in coverity_task same as in linux_task

Cirrus CI: Set LANG in coverity_task same as in linux_task

To avoid these warnings:

perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
        LANGUAGE = (unset),
        LC_ALL = (unset),
        LANG = "en_US.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").