All, or at least most, of our debs contain "./" as a folder. This
causes problems when extracting on some systems, as ./ then is the
system root directory /, and tar cannot change the permissions of this
folder. Trying to build on arch for example gives:
tar: .: Cannot change mode to rwxr-xr-x: Operation not permitted
tar: Exiting with failure status due to previous errors
The issue appeared on arch somewhat recently, maybe with tar 1.33. To
avoid having to rebuild all packages we now handle both data.tar.xz
types, with prefixed ./, and without.
Also remove some indentation levels while we are at it.
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