mkosi: explicitly add mount package to build image

mkosi: explicitly add mount package to build image

Some dependency changed in testing and it's not pulled in anymore:

‣  Running build script /home/runner/work/systemd/systemd/mkosi/mkosi.images/build/mkosi.conf.d/debian-ubuntu/mkosi.build.chroot…
/work/build-script: line 35: mount: command not found
‣ /work/build-script failed with non-zero exit code 127
‣ (Maybe a program was not found or the script interpreter (e.g. bash) is not installed?)
FAILED: [code=127] mkosi