Set NO_PKG_MANGLE=1 when building dummy packages

Benjamin Drungrequested to merge
bdrung/sbuild:no-pkg-mangle into main

Ubuntu uses pkgbinarymangler [1] to strip translations from the source package since Ubuntu ships the translations bundled in language packs. pkgbinarymangler's dpkg-deb expects to be run from a source package. Setting NO_PKG_MANGLE=1 is only needed on Ubuntu, but does not hurt to be set on Debian as well.

[1] https://launchpad.net/pkgbinarymangler

Edited by Benjamin Drung

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