tar_em_up.sh: dpkg-deb configuration via environment variable

This allows the path and name of the dpkg-deb tool to be configured via the dpkg environment variable when invoking the tar_em_up.sh script.

This should rarely be necessary, but there's one important use case I need it for: disabling the Debian package build when invoking tar_em_up.sh via make, which runs tar_em_up.sh with automatic target detection (-T). With automatic target detection, tar_em_up.sh will try to build a Debian package on any Linux system if the dpkg-deb tool is available, no matter whether the system is actually Debian-based.

On the Manjaro system which I use to do my local package builds, I do have the Debian packaging tools installed, because I also create Debian source packages for the OBS builds there. But I have no use for the binary Debian packages that tar_em_up.sh will then create when building Purr Data locally with make, in fact the deb creation slows down the build near the end quite a bit, which means wasted time. So now I can just set dpkg to disabled (or anything else which isn't executable) when running make to force tar_em_up.sh to skip the Debian packaging.

Yeah that seems like quite a lot of hassle to just skip the creation of those unwanted debs. ;-) But I do save quite a lot of build time that way, that's why I need this.

On other platforms than Linux, tar_em_up.sh will just ignore this setting. On Linux, normal builds won't be affected either (unless you have the dpkg environment variable set, which shouldn't normally be the case).