doc: release: remove build dir before creating tarballs
doc: release: remove build dir before creating tarballs

certtools/intelmq#2116 revealed that not removing the build directory
may result in different files being packaged.

aaronkaplan/intelmq,aaronkaplan/intelmq,certtools/intelmq,certtools/intelmq,certtools/intelmq,aaronkaplan/intelmq