switch from dpkg -x to ar x && tar -xvf

[yarikoptic]

installing a standalone build is useful for

it is possible to extract without using dpkg and just using ar and tar which should be available on any of target systems (whenever dpkg - only on Debians):

(datalad) [d31548v@discovery7 8.20210803+git153-g63c2cfd47-1]$ ar x /dartfs-hpc/rc/home/v/d31548v/proj/datalad/ci/git-annex/builds/2021/08/cron-20210830/build-ubuntu.yaml-397-69466103-success/git-annex-debianstandalone-packages_8.20210803+git153-g63c2cfd47_amd64/git-annex-standalone_8.20210803+git153-g63c2cfd47-1~ndall+1_amd64.deb
(datalad) [d31548v@discovery7 8.20210803+git153-g63c2cfd47-1]$ ls
control.tar.gz  data.tar.gz  debian-binary
(datalad) [d31548v@discovery7 8.20210803+git153-g63c2cfd47-1]$ tar -xzf data.tar.gz 
(datalad) [d31548v@discovery7 8.20210803+git153-g63c2cfd47-1]$ ls
control.tar.gz  data.tar.gz  debian-binary  etc  usr

so I guess ar x should first extract into a tempdir and then tar could extract data.tar.gz into the target install-dir path, and then tempdir be cleaned up.